@crystalsu2001 It's possible you're running into a separate issue; you may want to open your own thread, using your actual domain name.
However, the limited evidence I can see is that the .cn TLD nameservers might no longer be replying to requests for everyone worldwide. You need to have working DNS servers worldwide in order to prove control over a domain worldwide in order to get a valid-worldwide certificate. Let's Encrypt checks from multiple places in order to help confirm that a requester is actually in control of the name and that it's not an attacker who can only impact some of the Internet. So if your domain name isn't working (from an external perspective), you won't be able to get a certificate, from Let's Encrypt or from anyone else that the authoritative DNS servers are blocking.
Some online tools that you might be able to use to see if your DNS servers are working externally include:
But those only test if your DNS servers are accessible from where that test system is on the Internet, not from everywhere like a Certificate Authority has to do.
If you're expecting that your domain name should be accessible worldwide, and it looks like it isn't, then I recommend contacting whomever it is that you pay for DNS service to look into it.