Hi Tian6,
Can I ask you to check for the extension from within Chrome? To do this, please copy and paste chrome://extensions into the Chrome address bar, and press Enter. Is Avast Online Security once of the tiles showing there? Is the extension enabled (the toggle switch in the bottom right corner of the tile should be colored blue)?
Please also be aware - we have recently changed the extension icon (please see attached images; the old icon was shaped liked the Avast logo, where the new icon is shaped like a shield).