This error message is displaying with the multi dropdown function. In the database, only “Auto” is saved.
What the meaning of LOOKUP? It appeared only after I tried to edit that window again. Not happening the first time.
UPDATE (many thanks to @fayewatson). The multi select dropdown was probably corrupted by erasing some fields link to that dropdown. To solved it, just create a NEW multi dropdown. Hope it help others.
Can you share a link to the app editor?