If you get an error when installing a Let's Encrypt SSL certificate on cPanel similar to this:

Verify your domain using the DNSchecker tool to ensure that it is pointing to your cPanel's IP address.

Once the domain is pointing correctly and propagation is complete, uncheck the mail.yourdomain.tld entry on the SSL list and re-issue the certificate:

If you get an Invalid response from http://domain.tld/.well-known/acme-... error, it may indicate there are some access-blocking rules in your website's .htaccess file. To fix it, disable the .htaccess file by renaming it, for example, to .htaccess_disabled. You can do so using the file manager.

If you get an Error creating new order: acme: error code 429 or too many failed authorizations recently, it means that too many attempts to install the certificate were made and the rate limit was reached. In order to fix this, you will need to wait 7 days before re-issuing the certificate.