Hey guys, back for help. Why isn't my kick script working when you go bankrupt in materials?

Your script checks Materials Value just once at the beginning. You should make a loop to check for the materials periodically:

local plr = game.Players:GetPlayerFromCharacter(script.Parent.Parent)
local CHECK_INTERVAL = 5 --every 5 seconds for example
while true do
    wait(CHECK_INTERVAL)
    if plr.leaderstats.Materials.Value < 0 then
        plr:Kick("You have gone bankrupt in materials!")
    end
end

It is however discouraged, as roblox has special value change listeners, which you should use instead:

local plr = game.Players:GetPlayerFromCharacter(script.Parent.Parent)

plr.leaderstats.Materials.Changed:Connect(function()
    if plr.leaderstats.Materials.Value < 0 then
        plr:Kick("You have gone bankrupt in materials!")
    end
end)

Better still, even if there is one player allowed, you should put script like this on server, to combat potential future exploiters. Following script should be in ServerScriptService:

local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(plr)

    --insert your leaderstats code here
    --code below is my best guess only
    local leaderstats = Instance.new("Folder")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = plr

    local materials = Instance.new("IntValue")
    materials.Name = "Materials"
    materials.Value = 0
    materials.Parent = leaderstats

    materials.Changed:Connect(function()
    if materials.Value < 0 then
        plr:Kick("You have gone bankrupt in materials!")
    end
end)
end)

P.S. My personal opinion is that you should not ever kick players for things like this. They will be discouraged from joining Your game again. Is it not better to simply respawn character?