What is wrong with this script?

One) Main Loop Placement

Lua is a procedural scripting language (on the whole). This means it executes in direct order.

The while loop from lines 04 to 07 will never stop. This means the function declarations below it will never be run.

The main loop must occur after all declarations.

Two) Tab & Space properly

Tab and space your code properly. It makes it much easier to read and understand.

time_left=(900) should be time_left = 900.

infected_finish should look like this:

-- NOTE THAT THE FOLLOWING CODE INCORRECT
-- AND IS BEING QUOTED FOR STYLISTIC PURPOSES
-- ONLY
function infected_finish(text,time)
	if game.Teams.Survivors:GetPlayers() == nil then
		m = Instance.new("Message", Workspace)
		m.Text = text
		wait(time)
		m:Destroy()
		for i, v in pairs(game.Players:GetPlayers()) do
			v.Character.Humanoid.Health = 0
			time_left = 900
		end
	end
end

Three) GetPlayers

Team objects do not have a GetPlayers() method.

Furthermore, if they did, it would never return nil. That would be a horrible design. A list would be empty, that is, have a length (# operator) of 0.

Here is a function you could use to get the players on a team:

function getTeam(team)
	local players = game.Players:GetPlayers();
	local teamPlayers = {};
	for _, player in pairs(players) do
		if player.TeamColor == team.TeamColor then
			table.insert(teamPlayers, player);
		end
	end
	return teamPlayers;
end