Help w/ function not running in a while loop?

Once problem is you're running PlayerAdded after a Player is already added, so that Player won't be affected(in the case you're testing it in studio). Also your script will only fire once(the loop) since you're not resetting it, it'll go below 0(-1, -2, etc).

Here's a example of what I did to make it jump all players(I explained the code I added):

function StartJumping(Player) -- Added a variable here
	local Player = game.Players:FindFirstChild(Player.Name) -- Look for the player
	if Player then -- In case it doesn't find it
		repeat wait() until Player.Character -- Wait til it finds the character
		while wait(.1) do
        	Player.Character:FindFirstChild("Humanoid").Jump = true
            Player.Character:FindFirstChild("Humanoid").Sit = true
		end
	end -- End the if function
end


Time = 20
while true do
    Time = Time - 1
    print(Time)
    wait(1)
    if Time == 1 then
		for _,v in pairs(game.Players:GetPlayers()) do -- Gather all the players
			StartJumping(v) -- Make all of them jump
 		end -- End the for _,v in pairs
    end
end