One reason an inquiry may not be processing is because of the checklist you are trying to process the inquiry with. If the first milestone on the checklist does not have a candidate status tied to it, processing the inquiry will throw an error.
To fix this so the inquiry does process, you'll need to add a candidate status to the first milestone of the checklist. To do this, you'll go to:
onBoard > Settings > Checklist
Edit the appropriate checklist
Edit the first milestone and make sure a status is selected under Candidate Status
Now you will be able to go back to process the inquiry form successfully.