"Insurmountable" means that something can't be overcome, as in an "insurmountable obstacle".
It is better to use "insurmountable." "Insurmountable" is the correct spelling and means something that cannot be overcome.
They are insurmountable and unsurmountable
Something which is "insurmountable" is something which can not be overcome - as in:- Insurmountable technical problems brought an end to the project.
An obstacle may seem insurmountable.
The mountain we tried to climb was insurmountable.
impossible
Despite insurmountable obstacles in his life, he had done very well.
impossible
Manageable.
something apparently impossible to do
"Insurmountable" refers to something that cannot be overcome or surpassed. It often describes obstacles or challenges that are extremely difficult or impossible to conquer.