Hi there! Sorry to hear you're having trouble logging into your MyADT app on your Mac. Let's work through this together.

First, to better assist you—could you let me know if you are seeing any specific error messages when you try to log in? Also, are you sure you're entering the correct email and password? Sometimes a small typo or caps lock being on can cause login issues.

Assuming you're entering your login details correctly, here are some easy steps to try:

1. Check Internet Connection: Make sure your Mac is connected to the internet without any interruptions.
2. Reset Password: If you're unsure your password is correct, try using the "Forgot Password?" option on the MyADT app login page to reset it. Passwords are case-sensitive, so make sure to enter the new one carefully.
3. Update the MyADT App: If you installed the app on your Mac (via simulator or related method), ensure it is up to date. Outdated versions can cause login problems.
4. Clear Cached Data: Sometimes cached app data gets corrupted causing login issues. If possible, clear the app cache or reinstall the app on your Mac.

If you mainly access MyADT through a browser, you mentioned a Macintosh OS X 10.15.7 system, so you can also try logging in via Safari or Chrome to see if the problem is app-specific.

If you've already tried these steps or still face trouble, please let me know any error details you see or what you've tried. We can dive deeper into the account status or check for wider technical issues.

How does that sound for a start? Are you able to try logging in now?