"User actions" search function on interface doesn't offer all fields

Unfortunately, Airtable will only allow you to choose a field if it visually appears somewhere on your screen.

So you will need to make that field visible on the screen somewhere, and then they’ll be able to search on that field.

- ScottWorld, Expert Airtable Consultant 

The field that I want to let them search/filter on is the primary field.

Ah, I never noticed that before, but the “User Filters -> Elements” option only lets you select certain types of fields. No single line text fields are allowed.

However, if you’d like them to filter on a single line text field, you can enable the filter for the entire page in “User Actions → Filter”.

- ScottWorld, Expert Airtable Consultant

Not fully related, and different use case, but to Scott’s point what I’ve done when I have a non-filterable field which (i) I want to filter by; and (ii) has a fixed amount of options (e.g. A date time format formula for showing months), I have the formula output results as single select and create all options in advance! In that way I can now have the filter on the interface.