I have doubt that it may possible that the input field is in repeating group or in reusable element that is why it is not showing in the workflow option. Anyway, you can check the below things

  1. Reload the editor, sometimes due to bubble’s own bug editor doesn’t work as expected, refreshing the page works fine in many times.
  2. Are you adding the workflow in some reusable element? if yes, then it needs to be processed differently like some custom state.