It appears to be a painting of Pavlo Shandruk (Ukrainian: Павло Шандрук, Polish: Pawło Szandruk).
As Pieter noted it is a mid 20th century fascist German uniform in style.
As Luiz noted the cockade and neck insignia are off, and appear Ukrainian National Army (UNA).
I then researched UNA (Luiz) generals (based on my observation and Pieter's confirmation). Wikipedia's image of Pavlo has the same face and moustache.
This image is the apparent source of the painting. The painting has been colourised. Shandruk's Waffen SS division, later UNA division, was a "waffen grenadier" division, potentially leading to borrowing the panzer grenadier colours. The image also emphasises political legitimacy due to swapping a cigar with a paper, and humanises by bringing eyes forward. The painting appears to remove some of the affability of the photograph, rendering Shandruk far more serious. However, the artist's lack of ability may be responsible rather than intent here, see the eyes.
The German fascists organised the UNA as an "independent" state. The UNA were a puppet nation with no territory. In keeping with this Shandruk's cockade is Ukraine not German, his neck tabs have a Ukraine nationalist insignia, and as the UNA was "not part of the German state," and so lacks the Eagle and Swastika insignia. This actually makes the painting more fascistic in intent (along side verisimilitude to the photographic original) in that the painting is not an appeal to mid-20th Century German fascist nostalgia, but an appeal to late-20th Century and 21st Century Ukrainian fascist mobilisation.
The distinction without difference of the UNA from its precursor Waffen SS division was a late war nicety to try to get a few more months of combat from fascist organised combatants by appealing to their national & racial imaginary, rather than the German national & racial imaginary. As such this image is well placed to appeal to contemporary Ukrainian nationalist & racial myths.
To a certain extent the German fascist appeal failed: Shandruk's division achieved internment as opposed to being destroyed in the field. Shandruk was successful in making a curious argument to the Western powers for his former waffen ss division members being treated as Polish citizens, rather than people to be transferred to Soviet power. Shandruk has pre-War political history of some length (as expected of a General officer in the East in 1945).
As such the painting appears to be fascist friendly modern Ukrainian propaganda, in a modern reinterpretation of Ukraine's fascist pasts.
Although there are some discrepancies, this appears to be a General der Infanterie of the Panzer Grenadiers, wearing a general officer's version of the M36 wool greatcoat over his service dress (though perhaps in less than full formal attire shown below).
Note that while in late war Whermacht feld grau uniforms all tended to a dusty grey colour due to cost savings introduced in 1940 and again in 1943,uniforms prior to 1940 possessed a distinctive greenish hue. As German officers were responsible for procuring all their own uniforms, not only were those uniforms typically much better tailored than for lower ranks but officers of better means would continue to procure and wear pre-war quality materials. In highlighting the green, as here, the artist seems to be emphasizing the financial means of the subject (and perhaps also his vanity).
Quotes below otherwise unattributed are from Chapter IX - Uniforms, Insignia, and Individual Equipment of the U.S. War Dept.'s Handbook of German Military Forces.
The red facing of the great coat is reserved to general officers. The "dark, bluish-green imitation velvet" collar I not only believe to be unique to the German Heerbut identifies the coat to be the M#^ style.
The double-breasted six-button, wool-rayon overcoat is standard for all ranks, except that general officers wear gold buttons and have red lapel facings (and administrative officials in general-officer grades wear dark green facings). Collars, once of dark, bluish-green imitation velvet, now tend to be plain field-gray wool. The coat, which is cut narrow at the waist, flares at the bottom, and has two side slash pockets. The ordinary leather belt may be worn, run through slits on the side so that it runs inside the rear of the overcoat without interfering with the cloth belt at the back. Overcoats have degenerated in quality of material in the same manner as the field coats.
This particular style of peaked cap i again believe to be unique to the German armed forces. Note that the gold (rather than silver as noted below) cord around the brim again identifies the subject as a general officer.
The Wehrmachtsadler, or Wehrmacht eagle, that should be on the cap's peak is missing. This would be inexcusable for a Wehrmacht general officer - but is also a forbidden symbol in several European countries. The likely means that thee painting is from a time after those laws came into effect.
The service cap is similar to the U. S. Army service cap, but is upswept to give the wearer the appearance of height. The visor is black, with a silvercorded chin strap for officers, and a black leather strap for noncommissioned officers and men. The cap band is of dark, bluish-green imitation velvet (blue-gray for Sonderfuhrer), piped top and bottom in the color of arms. Piping also appears around the crown of the cap. The cap cover is field-gray. The national emblem (an eagle, stylized differently for the different Armed Forces and Party organizations), and below it the national colors (black, white, and red) surrounded by oak leaves, are worn on the cap front. Officers often wear service caps in the field.
The grass green background of the subject's shoulder boards is the color for panzergrenadiere - German armoured infantry - troops. The bright red background for the collar patch as well as the intricate gold braiding are again unique to general officers.
However, the shoulder boards worn in the picture are clearly non-standard, other than the apparent double pip that distinguishes the various General der Waffengattung ( General der Infanterie, Artillerie, oder Kavalerie), all equivalent to a NATO three-star Lieutenant General.
Colors are the basic identification of arms (there are no branches in the German Army). Color of arm is usually to be found on shoulder straps, on the service cap, on old-style field caps, and on collar patches of line officers' field uniforms, and on collar patches of service and dress uniforms. Piping in the color of arm appears on service and dress coats and trousers.
It may be a German General der Infanterie of the Panzer Grenadiers as Pieter Geerkens answers. The painting may be inaccurate regarding military insignia. And the style looks very Third Reich-like.
the shoulder insignia does not look German
the cockade does not look German (no red ball inside)
the neck insignia also does not look German.
Then I have another (more unusual) hypothesis:
The rank insignia, appears to be from Ukrainian National Army. As they were a Ukrainian unit fighting for the Axis, it would make sense to have German uniforms.
The blue-yellow cockade, it may be a Ukrainian cockade.
If somebody could find what a collar patch from a UNA general looks like, it would be another possible evidence.
It would be helpful if you have more info about your painting history.
BTW, that site http://uniforminsignia.org/ is wonderful to check these issues.