In your system group instead of system TimeLeft = 0 use system TimeLeft <=0
And as action instead of wait 999 seconds, system go to layout 2 for a debrief there for example.
If you want to "pause" the spawning of animals, etc once the timer is at 0, create a global variable (example "GameState" as a text. When the game is on set it to "InGame", when the time is over set it to "TimeUp").
You'll have to add a condition to the spawning of animals to check the value of this global variable.
Example for "Sheep Spawn", event 11 add to system every 10 seconds another condition (right click "Add another condition") system GameState = "InGame".
When TimeLeft is <= 0 set GameState to "TimeUp", this should prevent animals from spawning.
You can also add the condition in the group shooter to prevent the movement of the shotgun and more shot to be fired.