Central Contract with Item Category as Material Group is real feature which was long awaited where at the time of WK creation we don't have to give line item with qty and value. Nice Blog, super features. Thanks

Check config for plant, may be source list is mandatory for plant. Don't remember TCodebut in SPRO-MM-Purchasing-

There will be NO impact on invoice if your PO has GR-Based-IV.

If the delivery completion indicator is set for partial delivered PO, System doesn't stop you from taking GR for remaining qty.Either you should use latest GR possible date (along with error msg set). OrChange the PO qty to delivered qty. ME22N