You can also try this:
- uninstall the app
- turn the device off and back on (to reset background processes)
- re-install the app from Google play
- 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"
- 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