Under a user with the owner role, SpamExpert admin panel in Professional SpamFilter > Manage in SpamFilter Panel shows the following error:
PLESK_ERROR: Error: Access denied.
However, if logged in as a customer, the panel is available.
This is Plesk bug with ID PPPM-7270 which will be fixed in future Plesk updates.
Access SpamExpert management panel using customer's credentials, not credentials of a user with owner role.