I struggled for a while trying to use the Companion app for the Stream Deck until I found out how to correctly disable the Stream Deck App. If you don’t disable the Stream Deck app, then this app has control over the Stream Deck and Companion App doesn’t work.
I am running on a PC so to disable, go to “Task Manager” and find the “stream deck” app in the list and right click and “end task”. Now the Companion App should work.