Insurmountable 

Impossible to surmount; insuperable.

Hope thinks nothing difficult; despair tells us that difficulty is insurmountable. --I. Watts.

Insurmountable by Miles Flint March 31, 2004

Unsurmountable 

As 'insurmountable' but relating to a person (or persons)

We would have made the peak of the climb, alas the ineptitude of one member of the group made the task unsurmountable

Unsurmountable by HooseOfWidnes March 24, 2022