I’ve got the same problem in UE 5.6.0
Sadly those steps didn’t help in my case.
I followed the steps exactly as written on the tutorial page 3 times in a row now and still the same issue.
Funny enough, the exact same steps work in UE 5.5.4
Except that you can’t create the same project as in the tutorial…
I think the problem is that it isn’t even possible to actively select the AdventureGameMode (c++ version) and from there it spirals to the not even listed BP version which got created from the defective c++ version…
But the tutorial only says: Tools → New C++ Class → template = Game Mode Base → build in visual studio (with unreal engine closed) → start unreal engine → deactivate Live Coding → Right Click the new C++ Class → Create Blueprint class based on AdventureGameMode (or whatever you named it 
)
There is no modifying the cpp/h files in those steps yet, and as I tested and said before in 5.5.4 it works exactly that way.