You will run into a problem with running the automation with manual record creation. When a record is created manually, it is created without any data in any fields.
It is difficult to tell why you are getting the error. Have you tried re-testing the automation starting with retesting the trigger and retesting every action in order? Are you using a “when form submitted” trigger and does the field appear on the form?
@kuovonne, and for anyone else interested! Turns out that I was editing my automation while it was turned on which required you to explicitly “Update” the automation after you have edited it. This was not clear to me, the UI for “updating” your edited automation could be made more obvious IMO!