SAP Message C2224 - The reference operation set & you selected is not valid

Diagnosis
The reference operation set you selected is not valid.

System Response
The reference operation set will not be transferred.

Procedure
The reference operation set does not meet the selection criteria.
Check whether:
o The reference operation set is valid for the date
o The lot size is within its validity range
o The reference operation set is released for production
o The header unit can be converted to the order unit of measure