How do i make a npc when the npc dies the team yellow die?

You should use the .Died event of humanoids, which will trigger a function when the npc dies. You have to "connect" it to a function. Info on events / connections

Also, "game.Teams.Yellow.Players" does not make any sense, as "Players" is not a property of a team. However there is a function :GetPlayers() which will do this. It gives you a list of players, so you have to kill each player in the list using a loop. Unfortunately your simplicity does not work lol. Info on Generic for loops

function die()
    --"pairs" loops through this table
    for _,player in pairs(game.Teams.Yellow:GetPlayers()) do
        --safety check
        if player.Character then
            --easy way to kill
            player.Character:BreakJoints()
        end
    end
end

--connect npc death to your function
script.Parent:WaitForChild('Humanoid').Died:connect(die)