Sounds like Arduino’s servers are having a bad day. A 500 error usually means something broke on their end, not yours. You can try a couple of quick checks:
Refresh or try again after a little while.
Restart the IDE just in case.
If you need a library urgently, grab it directly from GitHub and install it manually.
If others are reporting the same thing, then it’s definitely a temporary server issue. It should clear up once Arduino fixes it.
It can be globally error also due to cloudflare outrage.