Phrase &1 in characteristic &2 not contained in phrase set

What causes this issue?

This error can occur for the following reasons:
The phrase is not assigned to the phrase set with which the
characteristic can be assigned values. Therefore, you cannot use the
phrase to assign values to substances manually.
One of the value assignments maintained has a phrase without a phrase
set assignment assigned to it. However, you can only maintain this
value assignment manually if the phrase without a phrase set assignment
is no longer assigned to it.

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?

Assign a phrase with a phrase set assignment as the value assignment.

Error message extract from SAP system. Copyright SAP SE.