Am gettingthe below errors

What might cause this to happen?

Wjhat can i do to fix this?

Purchase Order Line: PO0311816, 10, 700511-IRGP
E: Item number does not exist.
E: Unit of measure EA does not exist.
Purchase Order Line: PO0311816, 14, 9533266
E: Item number does not exist.
E: Unit of measure EA does not exist.
Purchase Order Line: PO0311862, 14, 701187-IRGP
E: Item number does not exist.
E: Unit of measure EA does not exist.