Source UoM group & not identical to target UoM group &

What causes this issue?

The unit of measure group for the transaction material is not the same
as that of the target material. The unit of measure groups of both
materials must be identical.

How to fix this error?

Use another material that uses the same unit of measure group, or
maintain the material master so that the two materials use the same
unit of measure group.

Error message extract from SAP system. Copyright SAP SE.