Scheduled scan won't start

I keep trying to schedule regular scans on my desktop but they never start. My desktop never goes to sleep so that’s not the problem. Can anyone help? Thanks

Please provide more details.
Keep in mind that we can’t see your system.

Waht os/sp?
What exact version of avast?
How do you schedule a scan? (step by step or perhaps a screenshot)

My desktop never goes to sleep so that's not the problem.
do you use screensaver? ..... you may use screensaver scan