#Discord
I first encountered this bug while I was casually taking pfp challenges.
It's quite often to stumble upon a person who can't change their profile pictures. From what I can gather, it would be the problem with your Internet, Cache or the server having a little hickups.
I find that PC client is more prone to this bug than mobile. I advice you to use mobile version in case your PC can't change your pfp.
How to change pfp if it's not working:
1) Use another device
2) Restart PC client (Ctrl+R)
3) Use browser client
4) Clear cache (for mobile users)
5) Pray to Discord God for the server to be good.