Units of measure & / & are different

What causes this issue?

The stockkeeping unit contained in the material document to be
cancelled is not the same as the stockkeeping unit contained in the
material master record. However, a document can only be cancelled if
the stockkeeping units are identical.

System Response

In this case, the system will not cancel the material document.

How to fix this error?

To carry out the reversal, re-enter the given transaction using the
relevant reversal movement type.

Error message extract from SAP system. Copyright SAP SE.