Red "X" in resource icon.

Summary

A red "X" appears in the resource tree.

Content

There is a resource with a red "x" instead of the icon, in the documentation says that this happens cause the resource is not configured and can not be used correctly, and then put some possible reasons, one is the time zone is not configured correctly for this resource, and the other one is the calendar is not configured correctly for this resource. I checked both possible reasons, but is not the case, when I click in the red "X" it says "The resource must have an active user assigned" and I did it, but the icon doesn't change.