So this local script was initially in a model inside a folder in ReplicatedStorage, but I moved it to literally every service ever and still didn't work. This is the script:
local mouse = game.Players.LocalPlayer:GetMouse()
mouse.Button1Down:Connect(function()
print("Yes")
end)

3 comments
Local Scripts inside ReplicatedStorage won't run.
It is cloned into CurrentCamera.
The initial parent needs to be the Camera, not ReplicatedStorage. If a LocalScript is put in ReplicatedStorage, it will automatically fail to function because it has no client to run with. However, you CAN put it in ReplicatedFirst and expect it to run, since ReplicatedFirst activates upon game initialization.