1. Unable to add an FTP account for a domain: Button Add an FTP account is missing in Plesk > Domain > example.com > FTP Access.
2. Subscription example.com uses "example_plan" service plan.
3. In Plesk > Service Plan > example_plan > Resources > Additional FTP accounts is set to 0.
The restriction is set for creating additional FTP users in Service Plan used by the domain.
1. Go to Plesk > Service Plans > example > Resources and increase number of Additional FTP accounts.
2. Press Update & Sync to save changes and apply them for all domains assigned to a plan named "example_plan".