unachievable

adj

1. (not-comparable)That cannot be achieved (or only with great difficulty).

   “However, the limitations of a storytelling project such as this seems to be that the notion of validation is unachievable mainly because it falls short of the so-called 'Hitchens's razor': that 'that which can be asserted without evidence can be dismissed without evidence'.”

   “Driver training has yet to take place, making SWR's promise of entry into passenger service this year looking unachievable.”