Replenishment data for customer &1 blocked by user &2

What causes this issue?

You wish to run procurement for customer &v1&. The replenishment data
for this customer is already being processed by user &v2& and therefore
blocked.

System Response

The procurement run is terminated.

How to fix this error?

Check if your entry is correct. If it is correct, try again later.

Error message extract from SAP system. Copyright SAP SE.