Hi, please try this:
- open the Avast Mobile Security app
- tap the three-lines icon in the upper left corner to bring the menu
- in the menu roll down and tap on "Avast Account"
- now tap "Disconnect" and "Sign out"
- select exactly the same method for login you use on the page
https://my.avast.com (Login with Google, Facebook, or E-mail)
- follow on-screen instructions to link the app to your My Avast Account
Your device will then appear in the account.
A link to setting up the Anti-Theft:
https://support.avast.com/en-us/article/215/"