Since that’s the tv menu, what does Panasonic suggest to fix it?

Does it reappear if you temporarily disconnect the optical cable?

Is this recent behaviour, or was it always like this? Is it an older TV? What source are you playing from. Is this source connected through HDMI? Older TV’s (especially Panasonics, I notice) often do not send surround from HDMI connected sources to the optical connection.

It’s a Panasonic Viera plasma from 2012, model TC-P55GT50.

My previous soundbar was an Energy one. When I had Optical out from the TV to the soundbar, the TV’s Surround menu was active, and I could set it to 2.0 or 5.1 (despite the soundbar not having surround qualities).  

Using the Sonos. optical to HDMI adaptor, the TV’s Surround menu is greyed out - it does not see the Sonos as a 5.1 device, only 2.0.

I have a case open with Sonos, as they assured me before my purchase that I could get 5.1 with that TV model using the optical to HDMI adaptor, and clarified that if I used HDMI only, I could only get 2.0.

Same with the Blu-Ray player, same with anything from Firestick.

Had tech support on the phone last night for over an hour, they had nothing. Hopefully I’ll hear back today, I’m not very happy at the moment...

Sonos had told me (before I purchased). - 
Your Panasonic Viera TV TC-P55GT50-2 Plasma TV from 2012]can work with the Sonos Arc Ultra using an optical audio adapter. However, using the optical connection means you won't be able to control your TV with CEC or voice commands.
For 5.1 surround sound from your Amazon Firestick and Blu-ray player, ensure they are set to output Dolby Digital 5.1. The TV's optical output should be able to pass this through to the Arc Ultra. Keep in mind that Dolby Atmos requires HDMI ARC or eARC, which your TV doesn't support with optical

Yet I spend over an hour with Sonos support last night to no avail. They were supposed to escalate but did not. I called again, and they said “you’ll get an email in 2 business days.”

Someone on Sonos forum contradicts what Sonos told me -

Seems I’d need to get an HDMI switch and the HD Fury Arcana to solve my problem. I’m in this deep, do I have to go in further?

VERY VERY unhappy with Sonos right now.