The Duolingo English Test is now administered through a desktop app. This is a better, more stable experience for test takers compared to taking the test in a browser. The app provides a secure and reliable environment for the duration of the test.
You may still take the practice test using a web browser. If you encounter an issue during the practice test with your browser add-ons or plug-ins (such as Grammarly), these will all need to be disabled in order to successfully start and complete the test.
Instructions to add/remove add-ons or extensions in Google Chrome are listed here.
You can also get around many browser-based issues by using an incognito browser in Chrome.