Cannot access and manage IIS application pool settings at Domains > example.com > Dedicated IIS Application Pool for Website.
PLESK_ERROR: Error: Unable to manage the IIS application pool because all websites are using a single dedicated application pool
The server-wide policy is set to Always assign one application pool to each subscription at Tools & Settings > IIS Application pool > Global Settings.
This is a bug with ID PPP-45368 which is planned to be fixed in future product updates.
As a workaround use the following server-wide IIS Application pool settings in Plesk:
Go to Tools & Settings > IIS Application Pool > Global Settings:.
Set Place websites in a dedicated application pool by default and allow shared pools and Apply the changes.