You can use it in a sentence like:
"The manager was cognizant of the team's concerns."
There is no established noun form for "cognizant".
However, you could potentially use it as a noun in a figurative or abstract sense, like:
"He spoke with cognizance of the gravity of the situation."
But even in this case, it's not a standard or widely accepted noun form. If you want to express the idea of "awareness" as a noun, better options exist:
* Awareness: "He spoke with awareness of the gravity of the situation."
* Knowledge: "He spoke with knowledge of the gravity of the situation."
* Understanding: "He spoke with understanding of the gravity of the situation."