Think you’ll find that they’re different operating systems from each other ?

Older sub and newer generation arc 

Dont know if you can have the old sub work with the new arc ?

Have you unbonded or removed the Sub from the Playbar in the Sonos app first? The Sub cannot be bonded to the Playbar and Arc at the same time.

Not sure ? How to unbond the SUB ? But I have removed the playbar now ? And I used the option forget my system on the Sonos app. 

How to remove a Sonos Sub:

https://support.sonos.com/s/article/4806?language=en_US

How to add a Sonos Sub:

https://support.sonos.com/s/article/4805?language=en_US

This is the message I keep getting from the app, however as I indicated the sub is powered on, and is connected to my router via Ethernet cable. Any suggestions to have this resolved ? 

Were you able to unbond or remove the Sub from the Playbar?

Disconnect the Sub from the router and let it connect to your Wifi network wirelessly. Wait a few minutes to allow the Sub to connect to your Wifi and try bonding the Sub to the Arc again.

Hi thank you for your answers. I had to connect my playbar back to my tv. Unbounded / removed my sub from the room. Then I connected the arc again, but unfortunately still the same problem ? Same as the picture I am showing 

Ok. I have reached out to Sonos help on the chat. After all I didn’t unpair / unbond the sub from my old playbar. After their recommendation to do a factory re set on my sub. Which I successfully did, it was a child’s play to connect to the arc. 
 

indeed the answer was to unbound the sub from the playbar. THANK YOU Guitarsuperstar