So, what did I do wrong here?

You're putting the message inside of Player. It needs to go inside of PlayerGui.

Door = script.Parent 
------------------------------------
modelname="Taxi Driver"
------------------------------------

function onTouched(hit) 
        print("Door Hit") 
        local human = hit.Parent:FindFirstChild("Humanoid") 
        local message = Instance.new("Message")
        message.Parent = game.Players:GetPlayerFromCharacter(hit.Parent).PlayerGui
        if (human ~= nil ) then
                if game.Players:playerFromCharacter(hit.Parent).TeamColor==game.Teams:findFirstChild(modelname).TeamColor then
                        Door.Transparency = 1
                        wait(1) 
                        Door.CanCollide = false 
                        Door.Transparency = 1 

                else
                        human.Health = 0 
                        message.Text = "Player was not a Taxi Driver" 
                        wait(5) 
                        message:Remove ()

                end

        end 
end 

connection = Door.Touched:connect(onTouched)

local debounce = false

Problem

The message is not showing up.

Solution

You put the message in the player. You should put it in the PlayerGui to be client-sided.

Suggestions

playerFromCharacter is deprecated, meaning it is unfavorable but still fully functional., use GetPlayerFromCharacter. Also, remove the "connection" variable and just keep it as Door.Touched:connect(onTouched). :findFirstChild was changed to capitalize to :FindFirstChild. Local variables are also in favor to stop lag. You can also parent a new instance by adding it, like this: Instance.new("Part", workspace). And the debounce is not there in the beginning so I'll fix that. This is all very outdated.

Final Script (w/ Suggestions)

local Door = script.Parent 

debounce = false

local modelname="Taxi Driver"

function onTouched(hit) 
        print("Door Hit") 
        local human = hit.Parent:FindFirstChild("Humanoid") 
       local message = Instance.new("Message", game.Players:GetPlayerFromCharacter(hit.Parent).PlayerGui)
        if (human ~= nil ) and (debounce == false) then
if game.Players:GetPlayerFromCharacter(hit.Parent).TeamColor==game.Teams:FindFirstChild(modelname).TeamColor then
            debounce = true
                        Door.Transparency = 1
                        wait(1) 
                        Door.CanCollide = false 
                        Door.Transparency = 1 
                else
                        human.Health = 0 
                        message.Text = "Player was not a Taxi Driver" 
                        wait(5) 
                        message:Remove()
                end
        end 
end 

Door.Touched:connect(onTouched)

debounce = false

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