how do I find a screenshot that has date and time and shows Smart Scan did not find any threats?
A screenshot (as such) isn’t saved.
If you run a Smart Scan then you would get that information after the completion of the Scan.
However the Smart Scan as far as I’m aware doesn’t store this data in a log. I could be wrong but it is so long ago that I ran one, usually as a result of a question about it in the forum.
Avast being an active on-access antivirus would scan all active files as they are run. become active.