Why was my PNG file rejected for Quality Issue?

Apart from the obvious reversal of left & rights legs, PNG is for images containing transparency. Otherwise, submit as JPEG.

Read through your submission requirements.

https://helpx.adobe.com/stock/contributor.html

The legs are reversed.

And I agree about the legs. At first glance, at least, there’s some kind of optical illusion going on that makes them look reversed or that they are attached incorrectly to the hips.

Viewed at full size, some areas aren’t sharp, a little cleanup is needed, and the fret markers lack detail.

The legs are attached wrong… unless he’s kicking his right leg sideways to the far edge of the frame, in which case he’s got the wrong shoes on the wrong feet.  I make pictures with guitars too, and correct fret markers and evenly spaced guitar strings is a must. It’s extremely hard to fix manually with a Wacom tablet, so I built a few neck-fretboard-string vector image models in Adobe Illustrator. I just make them 3d, rotate them in 3d space until they fit the guitar just right, and then paste them in as a smart object. Then you erase the areas that the fingers cover.  Well worth the initial effort if you’re doing guitars a lot.

s this an AI-generated image?

It looks blurry when zoomed in to 100%.
The legs look unnatural.
If it is a PNG, please check for fringing.

Yo must pay attention to details.