A patient will not be able to submit a questionnaire if they have left a Require an answer to be provided question unanswered.
If you were to go into the editing view of your questionnaire template, and scroll through the questions you have created, you may see that some of the questions have been set to Require an answer to be provided:
The questions would be marked Required in red when the patient is filling in the questionnaire:
It is completely up to your clinic if you decide to set questions as Require an answer to be provided or not.
Please note if you do have it ticked for a question, it is compulsory for the patient to fill that question.
If that question is left unanswered, they won't be able to submit the questionnaire form.