Value & is invalid

What causes this issue?

You have selected the "Other combination" function. With this function,
you can scroll to any combination of characteristic values from the
general selections.
However, only values that already exist in the general selection are
allowed as characteristic values.

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?

You can display the allowed characteristic values by means of the
"possible entries" pushbutton (F4).

Error message extract from SAP system. Copyright SAP SE.