PO25 error message

Trying to change UOM on PO25 line 343 from EA to PK and getting the following error message...

Line {2} Agreement Line UOM PK must match GTIN UOM EALine {2} Agreement Line UOM PK must match GTIN UOM EALine {2} Agreement Line UOM PK must match GTIN UOM EALine {2} Agreement Line UOM PK must match GTIN UOM EA"Line{2} Agreement Line UOM PK must match GTIN UOM EA"

What is causing this and where do I go to fix it?

Thanks