d9780
Okay, my last try to describe the way to your destination ;-)
After select of „Multiple Users”, select the second user in the list.
This is not the owner profile/administrator, but your self-created 2nd user!
The you see a menu, where you can change to your 2nd profile = user.
You can also give the permission for Phone/SMS, installing apps, installing apps from third party sources and below of this you must see the switch for „Allow running in background”!
The penultimate entry in this menu is "Install available apps", the last is "Delete user".
If you don't see such entries, you are not using the current GrapheneOS version.