Alternative qty unit & cannot be converted into base unit &

What causes this issue?

The alternative unit of measure which you have entered cannot be con-
verted into the base unit of measure.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

There are two options:
Enter an alternative unit of measure which can be converted.
Go into customizing and define how the conversion between units &V1& and
&V2& should be carried out.

Error message extract from SAP system. Copyright SAP SE.