Stage Manager should be in the Control Center.
Here is the icon for it:
View attachment 2357734
Upon clicking it, look for this button.
View attachment 2357735
Then you turned on Stage Manager!
If you don't see it, check and see if your Mac is running macOS 13 or 14.
Edit: Judging that your Sonoma Mac is a 2010 MacBook Pro, it does not have a Metal supported Graphics Card, so therefore Stage Manager won't work on it. I am sorry, Stage Manager renders stuff with Metal.