Hi @yohanes.ai,
Sorry you didn’t get a response before now, here are some details on the error,
A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Review the suggestions in this Community Tip for Quick Fix Ideas.
I suspect something on the server may be blocking or rate limiting requests from Cloudflare IPs, are you able to check this and whitelist the IPs? Also, the linked tip has some other suggestions 