After going crazy over this for hours, here is the solution:
OH BTW this has nothing to do with Steam at all.
IF you cannot find certain online BP nodes like “cache achievements” or “read leaderboard integer”, the problem is that you’re trying to access this BP node from within a wrong class. And by wrong, I mean I have absolutely no idea why this doesn’t work, but it just doesn’t. Access from within Player Controller or Game State is prohibited for some reason.
SOLUTION: you can ONLY access these nodes from within certain classes like PLAYER STATE or the LEVEL BLUEPRINT. Many other classes - even custom classes - DO NOT WORK.
I have absolutely no idea why this is, but it probably has a very good reason why it works this way. Would be nice from Epic to mention this little detail somewhere… like in the official documentation of the BP node… just an idea…