What should I use for a script on a Type Writer Effect?

Use a LocalScript for a "Writer Effect" on the Client

Use a Script for a "Writer Effect" on the Server.

Depends what you want to do with it, if it's like local based stuff which you don't need information from the server to get that text , well to be honest you should always use a localscript for a type-writer effect unless it's not a client.

LocalScript: (put this insnide StarterGui)

game.ReplicatedStorage.RemoteEvent.OnClientEvent:Connect(function(txt)
    for i = 1,string.len(txt) do
        script.Parent.frame.txt.Text = string.sub(txt,1,i)
        wait(.1)
    end
end)

ServerScript: (put this insnide ServerScriptService)

game.ReplicatedStorage.RemoteEvent:FireClient(game.Players.Zetruis,"test")
game.ReplicatedStorage.RemoteEvent:FireAllClients("test")

be sure to insert a RemoteEvent insnide replicatedstorage

but honestly you just need this as a localscript inside startergui for it:

function typewriter(text)
    for i = 1,string.len(txt) do
        return = string.sub(txt,1,i)
        wait(.1)
    end
end

print(typewriter("test"))