Hi Venkata,
1.As per the error message first check whether 'MN' is valid unit or not, if valid then do go to source ` system right click and transfer global settings, then unit is available in SID table of the 0BASE_UOM.
2.Use the transaction CUNI and check unit 'MN' is available or not.
3.Check the table T006 for 'MN' unit is available or not.
Thanks,
Somesh.