How to tell if a player isn't in the game?

Trying to make it so when someone types a username in a textbox and that username/player isn't in the game, tell them somehow and return false.

Script I got so far that isn't working:

local Username = script.Parent

local function onEntered(enterPressed)
    if enterPressed then
        for i, v in pairs(game.Players:GetPlayers()) do
            if Username.Text == v.Name and not v.Name then
                Username.Text = "Player Not Found"
            end
        end
    end
end

script.Parent.FocusLost:Connect(onEntered)