Getting concatenate error with my script that for custom chat, why? help me!

Well one problem I could find right away is that the Client-Server communication actually gives 3 arguments and not the two you specified.

All client-server communications that are utilized with the Event object provide the player itself.

So you can remove the "PlrName" argument and just utilize the player's name, here's a rework:

Edit: Kirot is right, I did a big booboo on this one rip. As he mentions in a comment, we should've done "OnServerEvent" instead.

Send Button:

script.Parent.MouseButton1Click:Connect(function()
    local text = script.Parent.Parent.TextBox.Text
    game.ReplicatedStorage.Message:FireServer(text)
end)

Label: (Ignore this one, it was a failure RIP)

game.ReplicatedStorage.Message.OnClientEvent:Connect(function(player, text)
    local PlrName = player.Name
    script.Parent.Text = PlrName..": "..text
end)

Rewrite of Label (This one doesn't work either rip):

game.ReplicatedStorage.Message.OnServerEvent:Connect(function(player, text)
    local PlrName = player.Name
    script.Parent.Text = PlrName..": "..text
end)

As for the text issue, it should've just returned the player's name. Are you sure the Send Button is the secondary or first parent of the script?

What I mean is:

Secondary Parent:

script --> 1st Parent --> SendButton (This is how it's working right now)

First Parent:

script --> SendButton (How I think it'll need to work like)