When trying to add a domain from Plesk panel, you might receive an error as below.
QuoteError: ProtDir_IIS::update() failed: Add Protected Directory failed: columns siteId, path are not unique (Error code 1)
The above error means the protected directory was not synchronized properly.
Login to your server via RDP & access command prompt. Execute the command below.
Quote"%plesk_cli%\repair.exe" --synchronize-protected-directories-storage
The above command will synchronize the directories on the server & you should be able to add the domains now.