✓ Accepted answer
I assume you're using a server sided script because that error came from server script service.
You cannot access the PlayerGui of a player using a server script. What you should do instead is put that in a local script and tweak it a little. You also tried to get PlayerGui from a table named "players" which is a common mistake because :GetChildren() is often misunderstood
local player = game:GetService("Players").LocalPlayer
local textLabel = player:WaitForChild("ScreenGui"):WaitForChild("TextLabel")
2 comments
So where do I put the localscript?
anywhere that is a descendant of a playeri.e startergui or starterplayerscripts