How can I make a variable accessable from all scripts?

Use something called a BoolValue.

BoolValues are either true or false, now the script that I'm creating here is if the player does not own the house the player will get damaged.

Instance a boolvalue in the player like this;

game.Players.PlayerAdded:Connect(function(player)
        local leaderstats = Instance.new("Folder")
        leaderstats.Name = "leaderstats"
        leaderstats.Parent = player

        local BoughtHouse = Instance.new("BoolValue", leaderstats)
        BoughtHouse.Name = "BoughtHouse"
    end)

then you can access it from another script

script.Parent.Touched:Connect(function(hit)
        local Humanoid = hit.Parent:FindFirstChild("Humanoid")
        if (Humanoid ~= nil) then
            local Character = hit.Parent

            local Player = game.Players:GetPlayerFromCharacter(Character)

            if Player.leaderstats.BoughtHouse == false then
                Humanoid:TakeDamage(20)
            end
        end
    end)

or you can use modulescripts.