Incomprehensive is an adjective that means something or someone is either not comprehensive, or not all-encompassing. The word can also describe something or someone has having a lack of understanding, that the person is not able to quickly comprehend knowledge.
The adverb form is incomprehensively, and the noun form is incomprehensiveness.
This word is listed in some online dictionaries, but not all. It is used rarely and should be considered with caution. As with many fringe words, people may see it as an error.
Incomprehensible is much more common. It is an adjective that means difficult or impossible to understand, or comprehend. A good synonym is unintelligible.
The adverb form is incomprehensibly, and the noun form can be either incomprehensibility or incomprehensibleness.
Most of the time, a person who uses incomprehensive really should be using incomprehensible, like our first example below. An easy way to remember them is thinking of the base word of each, either comprehendable or comprehensive. The second example below uses the word in its correct sense, that the noun modified is not inclusive or covers everything.