"This script"? what script is that?
That error mesage comes when you use Relookup on a field that is not used as the key field in a relationship that is in turn used in a looked up value field option to copy values from a look up table. Since the field specified in the Relookup step isn't used in that manner, nothing happens except that you get this error message. You can eliminate this error message by eliminating the relookup step that is causing it.
Of course, you'll need to examine your script to see if the relookup should be referrring to a different field and see what values might need to be updated by this step and aren't being updated due to the step referring to the wrong field.
Thanks for that.
Modifying the RelookUp sorted the problem.