Task list &1 &2 &3 already exists

What causes this issue?

You want to create a task list with the group counter &V3& in the &V2&
group, in which you summarize task lists for task list type &V1&. In
the group &V2&, however, there is already a task list with the group
counter &V3&.

System Response

The task list is not created.

How to fix this error?

Create a task list with a group counter that has not yet been assigned.

Error message extract from SAP system. Copyright SAP SE.