Hi!
The passwords export file is a .csv format, which you can't import using the import button from bookmarks, as that option only supports .html file. If you have the file with your passwords exported from Brave, you need to follow these steps in order to import it to Avast Secure Browser:
1. Navigate to secure://flags.
2. Type "Password" in the search input box.
3. Enable the "Password Import" flag and click on relaunch.
4. Go to Settings -> Passwords -> Clicked on the 3 dots above the "Saved passwords" section -> Import. Choose the export file from Brave and all your passwords should be now imported