I'm having a similar problem: I had previously scheduled a full virus scan, and can no longer find any option to update that schedule. I can create a custom scan, but that seems to run in addition. So just to confirm: (a) the disappearance of the option to schedule a non-custom, full virus scan, is a bug; (b) the bug has been reported to Avast; and (c) at some time, which none of us know, it will be fixed.