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The experiment that was previously called Gametest was renamed to Beta APIs and is already enabled on your server. What the addon you installed wants access to is the GameTest JavaScript module, which is not available on Aternos, since it could be used to circumvent the automatic shutdown if your server is empty.
I also can't think of any reason why that addon would actually need access to the GameTest module, so I think it's just a bug in the addon that they left it in as a dependency. If you provide a download link for the addon in question, I can look into it.
