Have you created a workflow that has changed over time? Perhaps it started with different columns which some are no longer present. These fields may now display as yellow fields in your Select Tool. They may show up in your Results Pane with a warning when you run your workflow.
Did you know there is an easy way to quickly remove these fields?
Go to the Options drop-down in Alteryx on the Configuration pane of your Select Tool.
You have two options; one, where you can highlight specific fields and tell Alteryx to forget them, or two you can select that Alteryx to forget all missing fields.