/usr/local/directadmin/data/users/admin/user.conf

ERROR 
2021/10/28 05:31:30 [INFO] acme: Registering account for [email protected]
2021/10/28 05:31:30 Could not complete registration
acme: error: 400 :: POST :: https://acme-v02.api.letsencrypt.org/acme/new-acct :: urn:ietf:params:acme:error:invalidEmail :: Error creating new account :: contact email "[email protected]" has invalid domain : Domain name does not end with a valid public suffix (TLD)
Certificate generation failed.

FIX:
/usr/local/directadmin/data/users/admin/user.conf

da-cli –remove-user (username) Error letsencrypt