Welcome to the forum.
Have you had any other A/V products installed on this system before you installed Avast? Or, do you still have any installed on the system?
Sometimes when you remove Norton, AVG, McAfee, etc... they don't completely uninstall. Remnants of them will still be present on the system, and can cause conflicts.
You'll likely need to use a removal tool to remove the rest of the old A/V, then avast should work properly.