unrealizable

/ˌʌnriəˈlaɪzəbəl/
adjective
  1. Not able to be achieved, accomplished, or made real; impossible to realize.
    • The plan to build a city on Mars is currently unrealizable with existing technology.
    • She had to give up on her unrealizable goal of traveling to every country in a year.
    • His dream of becoming a professional athlete seemed unrealizable without proper training.