What is your brand and model receiver?
Verify with the receiver manufacturer if the receiver supports the Dolby MAT format that Apple TV uses. Early Dolby Atmos capable receivers may need a firmware update.
Play audio in Dolby Atmos or surround sound on your Apple TV - Apple Support
Apple TV: Settings﹥Video and Audio﹥Audio Format﹥Auto and
Apple TV: Settings﹥Video and Audio﹥Audio Format﹥Immersive Audio﹥Dolby Atmos﹥On
That option will only be available if that capability is detected. It won’t work in combination with changed formats Dolby Digital 5.1 or Stereo.
Apple TV HD, tvOS 14
‘Apple TV HD’ (A1625) doesn’t have Dolby Atmos functionality. You would need a 4K model: A1842 or A2169.