No planned order found for key 'plnum = &'

What causes this issue?

When backflushing with separate goods issue, the system creates
references to the reduced planned orders in table BLPR. The system
could not find the appropriate planned order for an entry in table
BLPR.

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?

Error message extract from SAP system. Copyright SAP SE.