Check to make sure the audio settings on the device you have connected to the TV is also set to send Dolby Digital. The show can be labeled as Dolby Digital, but if your cable box (or other device) is sending only a stereo signal to your tV, then the TV will only allow you to select stereo, and will only pass that signal to the Beam. 

I have nothing else connected to my TV. I am watching Netflix, Hulu and YouTube all through my TV. And it’s not allowing me to change it Dolby digital. 
I played a Dolby Digital Test Video on YouTube but to no avail

What TV is it? How is the Beam connected to it?

Ah I now see it’s a Samsung, sorry I missed that. Perhaps see if this “Toms Guide” link will help you resolve the matter…

https://www.tomsguide.com/us/samsung-tv-settings-guide,review-4663-9.html

It is a Samsung Tv connected through HMDI ARC but I have followed the Toms Guide already, but the option to switch to Dolby Digital is grayed our/unavailable although I’m playing a Dolby digital video. Is there a big difference between DD and stereo audio from the beam? Should I just settle for stereo ? 

You also need to be careful about which videos you choose. I’m not even certain that YouTube can even carry a 5.1 signal, I’ve just watched 3 different “test” videos, all were in Dolby 2.0. 

See the information here:

https://www.avsforum.com/forum/112-surround-music-formats/2253538-does-youtube-support-5-1-audio.html

You may need to test your surrounds using something other than YouTube.

Netflix usually shows the audio format alongside the movie title too in their app.

If no joy I would give Samsung Support a call and tell them you want TV to play Dolby Digital 5.1 audio out to the Receiver.

Note that the Dolby Digital Plus codec is not supported-so mind that is not chosen in error when you find out how to enable the Samsung menu option.