SAP Message REMPT401 - Conversion of unit of measure &1 to unit of measure &2 not possible

Diagnosis
You want to convert a material to a unit of measure that you have notmaintained for the material.

System Response
The system cannot change the unit of measure.

Procedure
Maintain the unit of measure you want for the material in the materialmaster record as an alternative unit of measure. (You enter thealternative unit of measure under "Additional data --> Units ofmeasure".)
Proceed with function