Humanoid is not a valid member of model... how do i fix?

Hello! The answer is the humanoid has not loaded yet. The character has, but it's still loading the humanoid, body parts, body color, etc.. It's a pretty easy fix though. Using :WaitForChild() you can yield the function (pause it) until it is found. If it has not found it within 5 seconds (which is the default), it will end the function.

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

local block = game.Workspace:FindFirstChild("Block", true)
local blockCount = 0
local blockTimer = 0

player.CharacterAdded:Connect(function(character)
    character:WaitForChild("Humanoid",5).MaxHealth = 1000 -- the 5 inside WaitForChild says how long to yield for until cancelling the wait, math.huge will make it yield permanently until it finds it
end)

mouse.Button1Down:Connect(function()
    blockCount = blockCount + 1
    local newBlock = block:Clone()
    newBlock.Name = "Block"..blockCount
    newBlock.Parent = game.Workspace
    newBlock.CFrame = player.Character.Head.CFrame - player.Character.Head.CFrame.LookVector

    if blockCount > 5 and tick() - blockTimer >= 10 then
        print(player.Character:WaitForChild("Humanoid")
        wait(1)
        player.Character:WaitForChild("Humanoid"):TakeDamage(1000)
        blockCount = 0
        blockTimer = 0
    end)

    delay(15, function()
        newBlock:Destroy()
        blockCount = blockCount - 1
    end)

    blockTimer = tick()
end)

game:GetService("RunService").Heartbeat:Connect(function()
    if tick() - blockTimer >= 10 then
        blockCount = 0
        blockTimer = 0
    end
end)

print(player)

Also, you are running this from a LocalScript. If this is a multiplayer game, it wouldn't kill you on everybody else's screen. You can look into PlayerAdded for a pretty easy solution on a normal Script. Also, I dont see the need for setting the health to 1000 only to deal 1000 damage. Setting the health to 0 would be a guaranteed kill regardless of ForceFields or hacks. Hope this helps!