Hi, first of all, please make sure you are using the latest app version, which is available on Google Play.
To find out the reason why the Anti-Theft locked your device please follow these instructions:
- login to your My Avast Account on the page
https://my.avast.com- click on the menu and select "Devices"
- click on the device with the issue
- click on "Info"
- now in the upper part of the screen select "Notifications"
- right now you should see notifications coming from the Anti-Theft
- right there should be the reason why the phone got locked
If there is any other problem you need help with, please send us some screenshots, thanks.