Hi Billy,
It seems the internal UoM mapping is missing for ISO code KGM. Check the UoM in your product master.
It should have been KG. While transferring to GTS, system picks up the ISO code for KGM from T006 in ECC and transfers to GTS. In GTS, system will try to pick up Internal UoM for ISO KGM from T006.
Regards
Dhilipan