Not possible to carry out conversion

What causes this issue?

The system cannot carry out the conversion required because the
alternative unit of measure you have entered cannot be converted to the
base unit of measure.

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?

Specify the conversion factors for the alternative unit of measure in
the corresponding material master record.

Error message extract from SAP system. Copyright SAP SE.