What causes this issue?

In master data maintenance for the asset, you have entered a
quantity, but you have not entered a unit of measure.
The asset cannot be managed with a quantity if the the unit of
measure is missing. In particular, it is not possible to enter
quantities when posting.

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?

Enter the unit of measure. If the field for the unit of measure
does not appear on your screen, or it is not ready for input, this is
due to screen layout rule &V1& in your master record.
In this case, you first have to end the master data transaction, and
change the screen layout &V1& in Asset Accounting Customizing.

Error message extract from SAP system. Copyright SAP SE.