Unable to install WordPress: Element ‘db-server-id’: ‘null’ is not a valid value of the atomic type ‘id_type’.

Symptoms

• In an attempt to install the WordPress in Plesk > Domains > example.com > WordPress > Install, the following error is shown:

  PLESK_ERROR: Parser error: Request is invalid. Error in line 1: Element ‘db-server-id’: ‘null’ is not a valid value of the atomic type ‘id_type’.

• The domain example.com is assigned to the Example Plan service plan.
• The MySQL is set to None in Service Plans > Hosting Plans > Example Plan > Hosting Parameters > Default Database Server.

Cause

This behavior is expected: WordPress Toolkit cannot create a database for WordPress instance when there is no database server specified.

Unclear error reporting is confirmed as a bug with ID #EXTWPTOOLK-4810, which is planned to be fixed in product future updates. 

Resolution

1. Log into Plesk.
2. Go to Service Plans > Hosting Plans > Example Plan > Hosting Parameters > Default Database Server.
   
3. From the drop-down menu MySQL select the available database server:
   
4. Press the OK button to apply the changes.