To determine why you are unable to create a new subsite in SharePoint, the following steps may help:

1. Check permissions:
   Make sure you have the correct permissions to create a subsite in SharePoint.
2. Check pre-existing subsite:
   Check if there is a pre-existing subsite with the same name or url you are attempting to create.
3. Review settings:
   Review any settings that may be blocking you from creating the subsite.
4. Check for special characters:
   Make sure there are not any special characters in the name of the subsite you are attempting to create.