First of all.....Welcome to the Cisco Learning Network!
Regarding your question of taking the CCNA Security exam.
Let me provide you with the short answer: Yes, you can take the exam.
But you will not be considered CCNA Security certified until you fulfill the prerequisites in order to obtain your CCNA Security certification. Meaning you need to have at least the CCENT certification or the CCNA Routing & Switching certification or any CCIE certification.
Also you can verify the prerequisites for the CCNA Security certification at the following link: Security (CCNA Security)
I hope this information is helpful for you and I wish you all the best in you CCNA Security studies.