Can't enable Vulnerability protection feature in Plesk: Vulnerability protection limit reached

Symptoms

Unable to enable Vulnerability protection feature for all WordPress domains at WP Toolkit > Security > Vulnerability protection > Protect all with the following message:

Vulnerability protection limit reached: you are trying to enable vulnerability protection on X WordPress installation(s), but your current plan does not allow you to enable vulnerability protection on Y of these WordPress installation(s).

Cause

Service Plan limits for one or several involved WordPress domains does not allow enabling Vulnerability protection feature.

Resolution

Change Service Plan limit for the related Subscriptions:

1. Log into Plesk
2. Go to Service Plans > service_plan_name > Resources > WordPress websites with vulnerability protection
3. Increase the value or set it to Unlimited
4. Press Update & Sync

In case the Subscription for the affected domain is locked:

1. Log into Plesk
2. Go to Subscriptions > example.com > Customize (on the sidebar to the right) > Resources > WordPress websites with vulnerability protection.
3. Increase the value or set it to Unlimited
4. Press OK

Note: by default WordPress websites with vulnerability protection value is set to 0 for all Service Plans.