Do you have Anynet+ (CEC) enabled in your TV settings?

Yes I do have Anynet+(HDMI-CEC) enabled

Do you have any other devices connected to the TV? 

I have a cable box and that works fine. 

Be sure you have the latest firmware installed on your TV. Then disconnect everything from the TV including the Arc. Unplug the Arc and TV from power for about a minute. Plug them both back in but keep the cable box and Arc disconnected. Be sure Anynet+ and eARC is enabled on the TV and redo the TV Setup process in the Sonos app. Connect the Arc to the TV when the app instructs you. When the Arc is connected correctly to the TV and the Arc is playing audio, reconnect the cable box.

If this doesn’t work, try the same process with eARC disabled.

Thanks. I tired the steps. But still I am getting the same error that the Arc mode is not connected. In my tv sounds settings I only see TV Speaker, Bluetooth and optical but no sonos

However I do get the tv sound from the Sonia Arc (this was happening even before) but when I check the eArc hdmi it says not source connection and plus the atmos option is unavailable to select in the sound settings 

Are you not able to change the Sound Output setting to “Receiver (HDMI)”?

I can switch to the hdmi source but it does not identify the Sonos arc

Can you post a photo of your TV’s audio settings?

Change Digital Output Audio to “Pass-Through” and see if this setting allows you to enable Dolby Atmos Compatibility.

I cannot select pass through as it’s disabled 

Are you using the Sonos-supplied HDMI cable to connect the Arc to the TV? Does keeping the cable box disconnected make any difference?

Yes using the Sonos supplied hdmi. No difference with keeping the cable box disconnected. 

I’m stumped and out of ideas. The only other thing I would suggest is to double check that the Arc is actually connected to the TV’s HDMI ARC/eARC port.

Yes it is connected to the eArc HDMI. I will try connecting on a friends tv to rule out  if it’s an issue with tv or somos system 

I tried on a different Samsung tv which had hdmi arc port and still could not get the arc working. It keep saying the same error that arc mode for connected. I thinking it’s the problem with the arc. 

Maybe you should contact Sonos.

Did you find answer? Mines worked perfect for almost a year and now same problem

Same problem here. Any solution to this?

I’m having the same issues, Samsung blames it on sonos and sonos blames Samsung. Passthrough is also not allowing me to select. 

Most units will reboot after power is removed for a few seconds. When rebooting a SAMSUNG TV remove power for at least a couple minutes because you need to wait for internal stored energy to dissipate.

You wouldn’t believe what worked for me!
*** Try a different HDMI cable ***

I had the exact same issue on my new 75’’ Samsung QN90A. 
Nothing worked. 

Then I tried the HDMI cable that came with the Sonos Arc, instead of the other I had connected. Poof! Everything worked instantly. The TV notified me it detected a Dolby Atmos device, and everything was in sync. 
 

This worked for me!!! Unplug your tv and wait a few minutes to turn it back on then the tv will recognize the Sonos. 

Decided to use a high fangled HDMI cable and couldn’t get the our Samsung 65” QN9DAA to connect with the Arc. After hrs of trying, I finally used the HDMI cable that came with Sonos Arc and instantly got a connection and everything worked like a charm as it was suppose to. I’m pretty sure it’s about the snug HDMI connection in the Arc HDMI port when using the provided HDMI Cable.