this is because you are trying to concatenate a number value, try a tostring() function for that

--LOCAL SCRIPT--

local RS = game:GetService("ReplicatedStorage")
local remotef = RS.RemoteFunctions:WaitForChild("Change3")

while true do
    wait()

local MV =remotef:InvokeServer(workspace:FindFirstChild(script.Parent.Parent.Parent.Parent.Parent.Parent.Name):WaitForChild("Humanoid").Health) -- The problem/error
if type(MV) == "number" then
    local strvMV = tostring(MV)
    script.Parent.Text = "[H] HEALTH: "..strMV  
elseif type(MV) == "string" then
    script.Parent.Text = "[H] HEALTH: "..MV
end

end

So the script above checks if MV is a number is a string or a number, if it is a string, it concatenates right away, else it turns it into a string and then concatenates it.

function cV (player,healthAmount)
    while true do
        wait()
    return healthAmount
    end
end

    game.ReplicatedStorage:WaitForChild("RemoteFunctions").Change3.OnServerInvoke = cV

the server script seems fine, although you really don't need that many while loops

You have

healthAmount.Value

If healthAmount is the same as Humanoid.Health, and not a NumberValue Instance then you don't need to add the ".Value".

Issues:

1. TOO MANY LOOPS
2. You don't need a server script for this
3. There are simpler ways to do this

Steps:

1. Place a LocalScript into StarterCharacterScripts

-- local script only

local plr = game.Players.LocalPlayer
local Char = script.Parent
local Humanoid = Char:WaitForChild('Humanoid')
local PlayerGui = plr:WaitForChild('PlayerGui')
local TextLabelToDisplayHealth = -- do ur thing here

Humanoid.HealthChanged:Connect(function(h)
    TextLabelToDisplayHealth.Text = h -- h is = to current health
    -- or 
    TextLabelToDisplayHealth.Text = '[H] Health: '..h
    -- or
    TextLabelToDisplayHealth.Text = '[H] Health: '..h..' / '..Humanoid.MaxHealth
end)

Use this code, modify to your favoring.

Hopefully this helped you

Best of luck developer.