OK - with those speakers you definitely do need S1 and S2.
I suggest that you power off your S2 system and uninstall the S2 app. Then look in the Sonos S1 for the Compatibility Checker Tool. Depending on what you have already done it may or may not be there. If it is there, run it. It will tell you that some of your speakers can be upgraded to S2, but if you refuse then it should give you the option to leave everything on S1.
If the Compatibility Check Tool is not there, just reset the S1 app (in App Preferences) and choose to connect to existing system.
Finally, power off all S1 devices, reinstall the S2 controller and connect to existing (S2) system.
Power S1 system back on.
I run a split system without this problem. What happens after “More Options”?
May I ask first which Sonos speakers you have in your S1 and S2 systems? Your profile mentions a gen 1 Play:5 - do you still have that?
If I had the speakers you list in your profile I would be running a single S1 system. Everyone has different priorities, but I am wondering whether your issue with the message stems from not having established the system split optimally. Before advising I would like to understand if where you are trying to get is the place you really ought to be.
On S1: 6x Play 1, play 3, Play 5
On S2: SONOS arc, mini sub, 2 roams
Under more options it doesn’t provide an option to exit. I uninstalled and reinstalled and the window went away but I’ve done that previously and it comes back eventually. I’ll get a pic of that when it happens again…
Before re-installing the S1 controller, power down the S2 units.