rejected for quality issue

Here's another example. Inspect by zooming in >100% and the edges become readily apparent.

It looks like you might be using some AI background remover, which is fine for personal use but not for commercial. The process of successfully removing backgrounds usually requires doing the cutouts manually.

If you create a black layer in the background of an image in Photoshop, the borders will look jagged.
You'll need to carefully process the edges.