Have you updated the firmware on all devices?

In System Update - check for updates - it says "no update required"

Or it should be somewhere else?

No, that’s the correct area. Your current sub is a gen 3, correct? I’d think it would have to be, seeing as you could see the option before, but it’s worth the question. You can only add a gen 2 sub to a gen 3, I don’t think (never tried it) it works the other way. 

You may want to call Sonos Support directly to discuss it. It would be a lot easier than a back and forth here in this forum.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

I’m seeing the option in the S2 App to connect a second Sub - having added a Sub (Gen 3) to the Home Theatre setup… I just don’t have a Sub to add to it.

You can check the version of the Sonos Subs in the System section of your My Account page.

What existing room configuration are you trying to add a second Sub?

“Connect Second Sub” option is only available for Home Theatre rooms. You cannot add a second Sub to a stereo pair/Sub etc

The existing configuration is a stereo pair/Sub.

I have not tried the Home Theatre… However I’m pretty sure this option was there in the stereo pair/Sub setup before.

I’ve not seen the option with a stereo pair and it’s certainly not a configuration that Sonos presently supports - two Subs apply to a Sonos Home Theatre only setup and one of the Subs must be a (Gen3) product.

To my knowledge, the option has never been available for a stereo pair, Sonos has, to this point, always restricted it to a Home Theater ‘room’, which I’d assumed we were working with, since you’d indicated that you’d seen it before. 

I concur, the option has only ever been seen for Home Theater.  As seen here:

https://support.sonos.com/en-us/article/set-up-your-sonos-sub

Add a second Sub to a Sonos home theater system

Your Sonos home theater setup can connect with up to two Subs. When adding a second Sub, at least one of the Subs must be a Sub (Gen 3). 

Dear All, thank you all the help. I configured home theater system with a Bar and two SonosOne and Gen3 three Sub. The option to "connect second Sub" appeared just fine. 

It doesn't work in a stereo pair. Thanks again, we can close this thread. Zoltan