Unable to convert between units of measure & and & for material &

What causes this issue?

No conversion has been defined for the given units of measure for this
material.

System Response

The system is therefore unable to convert between these units of
measure; processing cannot continue without such a conversion.

How to fix this error?

Define the conversion for these units of measure in the material master
record (transaction MM02). Go to the basic data view and access the
units of measure screen, then define all neccessary conversions for the
different units of measure required.

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

Error message extract from SAP system. Copyright SAP SE.