So I tried making a game with dialogs, and I wanted to make it after 5 minutes, you'll die. Here's the script that I used.
local dialog = game.Workspace.Map.Overworld["Security Guard"].Head.Dialog
local function onSelected(player, choice)
if choice == dialog.TalkChoice.WaitDeathChoice then wait(300)
game.Players.LocalPlayer.Character.Humanoid.Health = 0
end
end
dialog.DialogChoiceSelected:Connect(onSelected)
If you have any ideas on how you can fix it as it is not working, please let me know.

2 comments
Does it not kill the player? Does it give any errors in f9 console?
Try calling this through a remotefunction, that's serversided instead of localscript and should fix the issue.