Unit of measure & cannot be converted to &

What causes this issue?

In CO the cost element &V3& must be maintained so that quantities are
posted in &V2& or a unit convertible to &V2&.

System Response

The SAP System reminds you to make an entry in a unit of measure
convertible to &V2&, but allows posting with a different or no unit of
quantity. The posted quantity is updated in CO as a line item and not
as a totals record. A note is written in the totals record that a
posting has been made in an unconvertible unit of measure.

How to fix this error?

If possible, enter a unit of quantity convertible to &V2&.
Otherwise consult your accounting department to see whether quantities
have to be recorded in account &V3&. You define this in cost
center/order planning.

Error message extract from SAP system. Copyright SAP SE.