Unable to activate WP Guardian for a site: Vulnerability protection limit reached

Symptoms

Unable to activate WP Guardian vulnerability protection option in WordPress > example.com:

Vulnerability protection limit reached: your current plan allows to use vulnerability protection on 0 site(s). Upgrade your hosting plan to use vulnerability protection on more sites.

Cause

Subscriptions > example.com > Account > Service plan > [name of the service plan] > Resources > WordPress websites with vulnerability protection is set to 0.

A bug ID EXTWPTOOLK-11928 has been reported in order to improve the error message.

Resolution

1. Log in to Plesk.
2. Go to Subscriptions > example.com > Account > Service plan > [name of the service plan] > Resources > WordPress websites with vulnerability protection and enter a value higher than zero.
3. Click on "Update & Sync".