Reason

This error message is issued if Plan driven procurement process (External requirements) is activated in SRM . When plan driven procurement process is defined , SRM will be the leading system and PO cannot be placed in ECC system.

Resolution :

Follow the below step if this setting is to be deactivated.

SPRO --> Integration with Other SAP Components --> SRM

Delete all entries maintained in 

- Maintain Profiles for External Procurement

- Control External Procurement