Being able to mention a person in a task depends on our project permissions and also on the project and task permissions of the person we want to mention.
The first thing to take into account to mention someone, is that we can visualize it in the mentions dropdown and this depends on our project permissions. That is to say that after adding the @ in a task or rework and typing the name or scrolling, we will find the people who are members of projects to which we have access.
To understand this in detail, we can check Why don't I see a person in the dropdowns?
On the other hand, depending on the project and task permissions of the person we want to mention, we will be able to mention that person in any task. If that person has access to all the projects and/or tasks of the account (for example a Director or C-Level) we will be able to mention that person without any inconvenience.
On the other hand, if that person has reduced project and/or task permissions and can only view projects and/or tasks in which that person is a member, we will get the following error message:
To solve this, as long as we have task editing permissions, we can assign that person as a collaborator or follower of the task and then mention them. If we do not have editing permissions, we can ask our PM to add that person.