Unit of measure &1 is not allowed for the product

What causes this issue?

You have specified an alternative unit of measure for the product.
However, this unit of measure is not allowed due to the units of
measure already assigned to the product.
<ZH>Reason</>
Due to its dimension, the alternative unit of measure specified (&V1&)
can be naturally converted to a unit of measure (&V2&) that is already
assigned to the product. However, this natural conversion is possible
only using an additive constant. It is not possible merely with
conversion numerator and denominator.
For a product, the alternative units of measure that it is useful to
define are only those that can be converted merely with a conversion
numerator and denominator to all units of measure already assigned to
the product.

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?

Cancel the entry or specify an allowed unit of measure.

Error message extract from SAP system. Copyright SAP SE.