BillFromGstin should not be same as LocationGstin when SupplyType is 'P'
This error will receive when BillFromGstin has been mentioned same as location Gstin when supply type is 'P'
For that you need to check Location Gstin and BillFromGstin should not be same when supply type is of purchase.
While creating purchase document the Location Gstin should be same as BilltoGSTIn and not BillfromGSTIN.