Symptoms
Cannot issue a Let's Encrypt certificate:
Warning: The Let's Encrypt extension is no longer available. To issue SSL/TLS certificates, install the SSL It! extension.
Cause
According to Plesk Feature Deprecation Plan, Let's Encrypt interface was hidden. Its functionality is already included in the SSL It! extension.
Resolution
Use SSL It! extension to install Let's Encrypt certificates: How to install a Let's Encrypt SSL certificate for a domain in Plesk?