The Allow only secure FTPS connections option is set at the Tools & Settings > Security Policy page.
Connection via FTPS fails with one of the following errors (depends on FTP client):
Insecure server, it does not support FTP over TLS.
534 Policy denies SSL
Local policy on server does not allow TLS secure connections
Settings for FTP site were manually changed in IIS.
Note: This article can be applied only if you have administrator access to Plesk.
Contact a hosting provider if you do not have admin access to Plesk.
Reconfigure FTP site:
Go to Tools & Settings > Security Policy.
Set the Allow both secure FTPS and non-secure FTP connections option and press the Apply button.
Set the Allow only secure FTPS connections option back and press the OK button.