Phrase key & is not within valid range

What causes this issue?

You attempted to enter a phrase set key whose value is not part of
the external number range.

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?

Contact your system adminstrator to find out the valid number range.
You can also choose automatic assignment by not entering a value in
the key field.

Error message extract from SAP system. Copyright SAP SE.