my first guess is that you're not using a good resolution for your cover art?
If this isn't the case, could you give more context about why you can't upload a cover art? maybe a screenshot if an error appear?
Edit: or maybe the issue is that i checked your profile and saw naked ppl, maybe the issue is that you can't (and shouldn't) put this kind of image in a cover art of a game, just in case your game gets recomended for a random kid before being flagged as nsfw...