Planning material & is locked (assignment check not possible)

What causes this issue?

<GL:planning_material>Planning material</> &V1& (plant &V2&), which
belongs to material &V4&, the material that you are processing, has
been locked by user &V3&. The system has unsuccessfully attempted to
lock this material.

System Response

To avoid further delaying your work, the system will terminate its
attempts to lock the material and will not execute an assignment check.

How to fix this error?

Note that the operation you are processing has not been confirmed.
Therefore, you should execute an assignment check
<DS:GLOS.availability_check>(availability check)</> again.

Error message extract from SAP system. Copyright SAP SE.