Under the Arc’s settings, choose the Set up Surrounds option and follow the app instructions.

I've tried that a few times but get the error message shown in the attachments. Both Era 300s are shown in the app and are connected to Wi-Fi. 

You should not be giving your speakers the same names as each other.  Every Sonos speaker must have a unique name before you bond the Sub and surround speakers.  Leave your Arc as Living Room.  Change the names of your Era speakers.  Might as well call the Eras “Left Surround” and “Right Surround”, although if you called them “Donald” and “Kamala” it would make no difference.

Then go into the settings for the Arc and do “Add Sub” and “Add Surrounds”.

You will end up with just a single room called “Living Room”. (That is true even if you called the surrounds Donald and Kamala.  Donald and Kamala will disappear.  And that is not just wishful thinking on my part.)

Hi @dblakester 

Welcome to the Sonos Community!

Sorry to hear of the issues you are having with your surrounds.

To expand a little on what @John B just said, it looks like the two Era 300s are set up as a stereo pair - please unpair the speakers prior to adding them as surrounds, and they will appear as valid options for adding surrounds.

I hope this helps.

Thank you for your reply. It turns out the speakers were on separate VLANs in my network. Once that was corrected everything worked fine. Thanks again for your input!