Gamerule

In what way is it not working? Do you mean that you are not seeing confirmation that the gamerule has been changed to false? If so, that's working as intended as you have just turned off command feedback. Or are you still seeing command feedback after running the command "/gamerule sendcommandfeedback false"?

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any /gamerule is showing up as unknown command in the latest edition. 

@unknown - are you using an underscore in your command? Does it work if you leave that out?

Also that's a java edition screenshot