Unit of measure & does not exist

What causes this issue?

Unit of measure &V1& is not defined.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Correct the unit of measure.
You can use the F4 function to help you to find existing units of
measure.
It is also possible that you did not enter a unit of measure, although
the screen layout rule specifies that the unit of measure must be
entered in the asset class.

Error message extract from SAP system. Copyright SAP SE.