To Create Hub Site in SharePoint Online follow below steps
Crete SharePoint Site
I have created SharePoint Site URL: https://nrsharepoint.sharepoint.com/sites/Contoso
Now Open SharePoint Online Management Shell and run below commands
Connect-SPOService -Url "https://nrsharepoint-admin.sharepoint.com"
Make Sure you need to pass admin site url in my case it is “nrsharepoint-admin” while connecting using SPOService
It will ask your Credentials for the site.
Once you are able to login add below powershell commnad to register your given site as Hub Site.
Register-SPOHubSite -Site "https://nrsharepoint.sharepoint.com/sites/Contoso"
Once you run above command it will ask you Principals[0]: parameter value do not pass anything for that value.
After Successfully execution of commnad it will Convert or Connect Existing Site to Hub Site
Nilesh Rathod 