Making a question required is not necessary if you only have one question, or if your survey has multiple questions that do not relate. If you are setting up logic to show a follow up question, based on an earlier question's response, it can be a good idea to make that question required to proceed.
A required question will show in your post-purchase survey with a * next to it.
To make a question required:
Navigate to your Survey editor
At the bottom of the question, you'll see Required in grey, click Required so it turns orange, click Save changes
This will also show in the right side panel, under the heading 'General> Required' as checked