Why does my group only gear script not even work?

It's because GetRankInGroup returns the rank of the group they are in and you're trying to index a number. It also didn't work because LocalPlayer is nil on server scripts. It works in LocalScript only. And because PlayerAdded has a player parameter you can use that instead. LocalPlayer only works on LocalScript because it is a descendant of the player, like the PlayerGui, maybe GUIS inside there, the backpack, the character which belongs to the player but it's not a child of the player.

local group = 4309200 --//local variables are faster

game.Players.PlayerAdded:Connect(function(player)
     local Level_0 = player:GetRankInGroup(group)
     if Level_0 == 255 then
        local level_0 = game:GetService("ReplicatedStorage").Level0
        local clone = level_0.L_X:Clone()
        clone.Parent = player:WaitForChild"Backpack"
    end
end)