You use Subcontracting in PP/DS and a PDS is used for order creation at Subcontracting Location.
When creating a Purchase Requisition for the Product at Plant Location, the order creation fails with error "Activity Not Valid" (/SAPAPO/OM_ERROR 043).
Conversely, if you try to create a Planned Order for the Product at Subcontracting Location, the same error is raised.
At the Product View, the following popup may be shown in this case
The issue is caused, in general, by a program error in PDS transfer.
To solve it, you may
1) Implement note 1824590 PDS-EXP: Dump TABLE_INVALID_INDEX after PDS explosion
2) Make an initial transfer of the Subcontracting PDS from ECC to APO
3) Retest the issue