thank you for your reply
I have a Let’s encrypt ssl certificate on my vps, and I use certbot to manage Cloudflare api to renew it auto,I use the command “certbot renew --dry-run”, it runs well,no error.but I still receive 521 error when I use the full(strict) model.
my domain is sandartgift ,because this reply cannot contain link so you can add a “com” after my domain