External processing - Purchase requisition cannot be generated

Hello experts,

I am facing the following issue while trying to build external process for work order operation: at routing level, when setting the operation externally processed, I populated information in External Processing tab, ticking Subcontracting. Then I assigned a material to the said operation to have it in the purchase requisition / purchase order later.

Then at Work Order creation (planned order conversion), the following message is displayed: Purchase req. for activity/element xxxx cannot be generated (CN705). If I untick in the routing Subcontracting, and I create again a work order, this time there is no error message and the purchase requisition is created. What did I miss?

Thanks

Jey