Timer Script Doesn't Work

I made a Script that displays a timer on the top of the screen.
When I run it in Studio, it works properly:
robloxapp-20210813-1708178.wmv (161.6 KB)
but when I play then game (studio and player) its breaks, requiring me to reset for the time to update.
robloxapp-20210813-1708359.wmv (3.3 MB)

I am trying to make the script work ingame. If anybody can, please try to solve it. Thank you!