Same issue. I did try the steps listed in this thread from a few years back, to no avail, maybe it will work for someone else? https://us.forums.blizzard.com/en/d3/t/start-game-greyed-out-unable-to-join-game-only-available-to-party-leader/4211/4
Edit: decided to try uninstalling and reinstalling as a last-ditch effort, seemed to fix on launching the game for the first-time post reinstall, however upon trying to enter game message appears saying that: “Servers are busy, estimated wait time: 1 second.” currently waiting in queue, with no expectation of creating a private game… so I’ll just wait till its fixed. Gives me a chance to do some chores in the meantime. 