The Log in button is missing at Domains > example.com > WordPress.
When clicking the Log In button at Domains > example.com, the web-page reloads:
The administrative password is not assigned to the WordPress instance.
Set up a new administrator password for the instance at Domains > example.com > WordPress > Setup.
Note: Setting up a login for WordPress in Plesk will not remove any data. This operation only changes admin password or creates a new WordPress admin user in case there is no such user registered already.