1. Once the extension is installed in the Chrome browser, Canvas needs to be refreshed in order to register that LockDown Browser has been installed.
2. Log out and back into Canvas to refresh and reset the test attempt.
3. If the extension still does not work, review extension settings and confirm that LockDown Browser has been turned on. Refer to the "Manage your extensions" information at this link - https://support.google.com/chrome_webstore/answer/2664769?hl=en
4. Try reinstalling, see attached file for installing LDB on Chromebooks.