In the following lecture snippet, the term 'Ajna' is missing; but I am not sure, why?
Is there any specific reason, for which he did not mention it?
To open that Suṣumnâ is the prime object of the Yogî. According to him, along this Suṣumnâ are ranged these centres of distribution, or, in more figurative language, these lotuses, as they are called. The lowest one is at the lowest end of the spinal cord, and is called Mûlâdhâra, the next one is called Svâdiṣṭhâna, the next Maṇipûra, the next Anâhata, the next Viśuddha, and the last, which is in the brain, is the Sahasrâra, or “the thousand petalled.” [source]
There are seven principle Chakras in the body which are: Muladhara, Swadhisthana, Manipura, Anahata, Vishuddha, Ajna and the highest Chakra is called the Sahsrara Chakra. Swami Vivekananda in the above lecture has talked about every Chakras except the Ajna Chakra.