you want new niches right? you should be allowed to make new ones should not be showing this message... i suggest you wait for sometime... log out and then try again
https://productforums.google.com/forum/#!topic/blogger/ZVvtWa8onT0
http://blogging.nitecruzr.net/2014/08/problems-with-create-new-blog-caused-by.html
I have encountered this problem a few months ago when creating a few (5-10) blogs in less than 30 minutes,I just simply restarted my router (dynamic ip) and I would create 5-10 more blogs and then just repeat.