A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Hardik Purohit,
Thanks for writing back and updating with us. Apologies for the delay in the response.
As per the further updates, I would like to confirm whether the problem happens even with the new SharePoint Online sites. If yes, then I would like to let you know that the possible cause is Subsite creation feature is disabled in the SharePoint Online admin center:
So, I request you to connect with the SharePoint Online admin of your organization and ask them to check whether the Subsite creation is enabled:
Steps for the admin of your organization:
Open the SharePoint Online admin center> Settings> Scroll down completely and click on classic settings pageat the bottom> Scroll down to Subsite Creation and see whether it’s enabled.
If it’s not enabled, please ask the admin to enable it, so that you can create the subsites in your SharePoint Online sites.
Appreciate your time and efforts with us. Have a wonderful day!!
Best Regards,
Sophia