You have not supplied any information to go off, I would start by checking over the community tip for 521’s.
If you’ve read through all of the Quick Fix Ideas, and still have not found a solution, share more information about your set up. Are you using a managed hosting provider? If so, contact them about this issue. Does the issue persist with Cloudflare paused? Pause Cloudflare · Cloudflare Fundamentals docs. If so, it’s a problem with your origin web server specifically, and you’ll have to fix that first on your end.