TL;DR: (probably) No :(
I'm afraid I don't think there is some site like this, mainly because it is much harder to generate minimal example games for memory manipulation which resemble a real life scenario than in reverse engineering or debugging challenges.
Games are often quiet complex and feature multi threaded gameloops and complex engines which keep track of various property values.
Although there are games meant to be messed with (such as pwnadventure3), I'm afraid examples like these are rare.
I would like to encourage you to make use of the tutorials on youtube. Try to target a game with minimal / no security measures first (single player, maybe indies without a big engine backing them) and try to search for tutorials. When you find one, it will indicate the target (e.g. fix health, enable flying, ...) and you can try to do this yourself without watching the video in the first place. You may watch it afterwards and use it to streamline your workflow. Like with most reverse engineering related tasks, hands on experience is king.
You can always make the computer show you anything you'd like. The real challenge starts with finding vulnerabilities in multiplayer games, because this way your manipulation exceeds the scope of your own system. For example, one of the first things usually checked in game manipulation is whether the server performs sanity checks on manipulateable values such as loot, health etc.
For example, when the server accepts jumping packages from the client and does not check for sanity (e.g. that the character is grounded and didn't jump a millisecond ago) you could abuse this behavior to implement flying. If the game utilizes a thick client structure, it may even trust the coordinates send by the client.
You'll have to explore game functionality and find the loopholes. For example, if a game implements a quick travel system maybe there are checks missing whether you could actually utilize it at any given moment or whether you can just travel to locations which are currently unlocked. Basically there are always two kinds of checks: requests something from the server and show its answer, or disable / change the interface to render the functionality unavailable to the normal user.
At some point, you may even want to mess with anti-cheat systems which check memory sections for manipulation and implement other strategies such as network proxies. There is a nice youtube playlist of LiveOverflow trying to beat pwnadventure this way.
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