I'm unsure of what you mean by turn on Windows Smart Scan
I don't know what is asking you to turn on the Smart Scan. Avast has a Smart Scan.
The Smart Scan would neither be on or off, it doesn't have an on or off state. It is run by the user when they want to run it. I can't recall if when you install avast new or clean reinstall of the program it asked do you want to run it. But it shouldn't be asking to constantly run.
A screen shot of this notice to turn on windows smart scan would help.
Windows Defender when avast is installed, it should switch itself as a resident AV, but can be on as an on-demand scan. I don't know if this is where the confusion lies.