How to make a when all players in team dies will run next code?

Something like this should work?

Basically, your problem is that your event fires when ANY humanoid dies. You need to keep track of how many there are, decrement a counter when somebody dies, and when there are 0 remaining, the team is eliminated.

function checkEliminated(counter, team)
    if counter.Value <= 0 then
        -- team eliminated
    end
end

function beginRound()
    for _, team in pairs(game:GetService("Teams"):GetChildren()) do
        if team:IsA("Team") then
            -- Create a global counter of remaining players in a team
            local aliveCounter = Instance.new("IntValue", team)
            aliveCounter.Name = "PlayersRemainingCount"

            -- Get all the players in this team
            local teamPlayers = team:GetPlayers()
            aliveCounter.Value = #teamPlayers

            for _, p in pairs(teamPlayers ) do
                if p.Character and p.Character:FindFirstChild("Humanoid") then
                    -- When they die, decrement the global counter
                    local conn = p.Character.Humanoid.Died:Connect(function()
                        aliveCounter.Value = aliveCounter.Value - 1 -- Decrement counter
                        conn:Disconnect() -- Make sure that dying again doesn't decrease the counter until the next round
                        checkEliminated(aliveCounter, team)
                    end)
                else
                    -- If they don't have a character, or they don't have a humanoid, they're already dead
                    aliveCounter.Value = aliveCounter.Value - 1
                    checkEliminated(aliveCounter, team)
                end
            end
        end
    end
end