in my game i am trying to make it so the player can use a flying ship and so it only lets the player gets out on the certain level of the z axis i need help with this.

i am trying to make my game in unity and with a zero$ budget (because i have no money i want a little help with the problem of "in my game i am trying to make it so the player can use a flying ship and so it only lets the player gets out on the certain level of the z axis i need help with this."