On my dedicated server, I tried to use the /gamerule command to do the command "/gamerule doInsomnia false" and it just responds with "Incorrect argument for command" with red text and a line underneath the doInsomnia false.
I am currently running on Snapshot 19w09a and i've tested it with Singleplayer and its working perfectly on there.

did you install CustomGamrules ?

Yes and both mods are the latest version.

As I said in the original post, It works in SinglePlayer whenever I put it in my own game, but as soon as I try it in my multiplayer server it starts not letting me do anything to them.

Can you post the log ?

I can try, How much of the log would be needed?

latest.log
hope that works

Ok, that should fix the issue, I'll roll an update soon

Uploaded on curseforge.

Found the problem, in dedicated servers, the gamerule command is registered before the mod.

Glad you found the problem