Go to Settings → Network → HTTPS Filtering and make sure that the Filter HTTPS protocol checkbox is ticked.
Disable all ad blockers other than AdGuard.
If you are using any other apps that can control Internet traffic (such as antiviruses), go to Settings → General Settings → Advanced Settings and tick the Use redirect driver mode checkbox, and then click Save.
If you're using an antivirus, add all executable files from C:\Program Files (x86)\Adguard to your antivirus's exclusions. Then open Settings → Network → List of filtered applications in AdGuard and uncheck the checkbox beside your antivirus.
If you're using a less popular browser, add them to the list of filtered apps manually via Settings → Network → List of filtered applications by clicking the + icon there.