Unit of measure & is not allowed

What causes this issue?

The unit of measure you are using is invalid. Valid language-dependent
units of measure are found in tables T006B and T006C.

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?

Maintain internal and external language-dependent units of measure in
table T006. Check whether all internal quantity units are translated
into your logon language.

Error message extract from SAP system. Copyright SAP SE.