Project definition &2 is being processed by user &1

What causes this issue?

Project definition &V2& is blocked by user &V1&. Therefore,
appropriation request &V2&, which belongs to project definition &V2&,
cannot be processed now.

System Response

The system does not allow processing of the appropriation request, and
the project definition belonging to it, at the same time.

How to fix this error?

Wait until user &V1& has finished.

Error message extract from SAP system. Copyright SAP SE.