No activity found for assignment of an output or input product

What causes this issue?

The system could not find an activity for assigning a receipt element
or requirements element.

System Response

Processing of the order is terminated.

How to fix this error?

Check whether all receipt elements (items, co-products, by-products)
and requirements elements (components) in SAP R/3 are assigned to an
APO relevant operation or an APO relevant phase.
Check additional messages in the application log for integration to
gain more information on why the system was unable to make an
assignment.

Procedure for System Administrators

Technical information:
Internal order key (POSITION_NO) of the receipt or requirements element
in SAP APO: &V2& &V3&
Internal order key (LINE_NO) of the receipt or requirements element in
SAP APO: &V1& &V3&
If there is an "x" after the order key, this is a requirements element.
Otherwise, it is a receipt element.

Error message extract from SAP system. Copyright SAP SE.