How do I call the player's Humanoid?

You can't have Local player in a regular script.

also you are telling the int value to parent it to the character but immediately change it to player just so you know character is the player on workspace the player inside Players is the one handling the Gui, local scripts, stats and some other stuff.

you don't have to separate this two script you can combine them too.

game.Players.PlayerAdded:Connect(function(player)
        -- Level

        local level = Instance.new("IntValue",player) -- the object after the comma is going to be its parent so no need to call level.Parent = player
        level.Name = "Level"
        level.Value = 0

        --Stats
        local strengthvalue = Instance.new("IntValue", player)
        strengthvalue.Name = "Combat"
        strengthvalue.Value = 1

        local healthvalue = Instance.new("IntValue", player)
        healthvalue.Name = "Endurance"
        healthvalue.Value = 1

        local staminavalue = Instance.new("IntValue", player)
        staminavalue.Name = "Fortitude"
        staminavalue.Value = 1

        local modevalue = Instance.new("IntValue", player)
        modevalue.Name = "Energy"
        modevalue.Value = 1

        --Balance and Avail Stats

        local bal = Instance.new("IntValue",player)
        bal.Name = "Balance"
        bal.Value = 0

        local availstats = Instance.new("IntValue", player)
        availstats.Name = "AvailableStats"
        availstats.Value = 0
        
      player.CharacterAdded:Connect(function(char) -- a function that let's you know if the the character of a player has been added to the workspace
      	local hum = char:FindFirstChild("Humanoid") 
      	if hum ~= nil then -- checking if humanoid is not nil
        	hum.MaxHealth *= 300 -- this is just the same as calling it like this hum.MaxHealth = hum.MaxHealth * 300
        
		end
	end)
end)