Data blocked by & (Table &)

What causes this issue?

When maintaining a controlling area, all the tables affected are locked
in sequence before the transactions are carried out. This is for
consistency reasons. If a particular table (&V2&) cannot be locked
because it is being processed by another user (&V1&), no controlling
areas can be maintained and you move to the display function.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Contact the user locking table &V2& (perhaps you are locking the table
in a different session) to release the table for maintenance.

Error message extract from SAP system. Copyright SAP SE.