Hello @Srabon ,
By design, a site collection is made up of one top-level site and all sites(subsites) below it. In addition to, only the site collection can be set as hub site collection.
This means that you can only create sites under the hub site collection, but cannot create hub site collection and hub sites under the hub site collection.
More information, please refer to this article.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.