You will need to try to take the test again when you have a stronger internet connection.
You will have a limited number of chances to re-take the test using the same coupon. If you already used these chances, you will need to reach out to Duolingo's helpdesk. Look for a chat button on their website that says "help" or "support," explain that you had an internet issue, and ask if they can reactivate your coupon.