When trying to synchronize or switch a subscription between Service Plans the following error occurs:
The plan was not applied because of problems with the following functionality: Domain Service Web not exists: domain=example.com, id=10
Some of the domains of the Service Plan do not have web hosting, e.g. they are used for mail.
This is a Plesk bug with ID #PPPM-13136: "Unchecked 'Log rotation management' option in service plan causing unsynced subscriptions if they contain domains without hosting".
Until the fix is available please consider using the following:
Navigate to Service Plans > 'Service Plan name' > Permissions.
Select the "Log rotation management" check box and click Update & Sync: