External planned orders cannot be converted

What causes this issue?

You tried to convert an external planned order into a manufaturing order
or a purchase requisition.
Planned orders of this type first have to be handed over to detailed
planning and scheduling. During this handover, the system creates
planned orders and purchase orders that can afterwards be converted.
The handover can be done by using heuristic <ls>Hand Over External
Planned Orders</> (<lb>/SAPAPO/SYP_CONV_HEU</>) in Advanced Planning or
report <ls>Hand Over External Planned Orders to Detailed Planning and
Scheduling</> ( <lb>SYPP_HANDOVER_RUN</>).

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

System Response

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.