AwesomeFriendYT

So I am trying to make a game that involves two teams who try to get as many kills as possible in a given time. I want the time left to be displayed at the top and counts down but I'm simply not able to make a script for that. The main problem I'm having is I can't set the value of the countdown. I have no idea what I'm doing wrong so it would be great if somebody could help tell me how to set the TEXT UI using scripting. I'm sorry if you think it is a stupid question but I just started.