Is there a really high mountain that has never been climbed

Ojo del Salado in the Andes Mountains.

Lots of them in Alaska and BC. Most likely some in South America too.

Most of the Himalayas and Hindu Kush have been climbed.

There are many less famous and unnamed peaks that either have not been climbed or have no official first assent - even in the US. Most of these will be very far from the nearest road and will require a very long approach.

Good Luck