& cannot be used as alt. unit since resulting unit would be too long

What causes this issue?

You have entered an alternative unit of measure which, when converted
to the base unit of measure, would be too long to fit into the
corresponding field.
Example
You define <LS>G</> as the base unit of measure, <LS>KG</> as the first
alternative unit of measure, and <LS>TO</> as the second alternative
unit of measure. According to the table, 1,000 g = 1 kg, and 1,000 kg =
1 t. As a result, 1,000,000 g equals 1 t, but this value is too long to
fit into the corresponding field.

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?

Reset the alternative unit of measure.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.