My Apple TV4 does not give option to select Dolby Atmos

I purchased a Vizio sb36512-f6 sound bar that supports Dolby Atmos. I purchased a brand new Apple TV4 HD and hooked it into the HDMI input of the sound bar, and connected my TV to the soundbar's HDMI out. The soundbar sounds good, but I'm not getting Dolby Atmos. In research, I get these instructions:

On your Apple TV 4K, go to Settings > Video and Audio and select Audio Format. Under Immersive Audio, check that Dolby Atmos is on. If you don't see Immersive Audio and Dolby Atmos as an option, you might need to change how your home entertainment system is set up.

Well... I only get an option to select "stereo" or "Dolby Digital 5.1" There is no option for Immersive Audio or Dolby Atmos. The instructions state If you don't see Immersive Audio and Dolby Atmos as an option, you "might need to change how your home entertainment system is set up". What kind of instruction is that?! What do I need to change? I've got Apple TV connected to the HDMI input on the soundbar, and I have an HDMI cable connecting the HDMI output on the soundbar to the TV's HDMI input. I've got it hooked up correctly! Why don't I get an option to set the Audio Format to Dolby Atmos????!!!!! Anybody with the right answer that actually works?