Follow the tutorial step 16 that present how to share the user between website.
If I understand correctly the procedure that you have applied your error is located in the "installed joomla in the new subdimain".
If you follow the procedure in tutorial, you will see that you can NOT install joomla in the new domain and that you have to create a rule for the replication of a website in which you decide to share some content.
If you install Joomla in a sub-domain, as the user are already created, you can not replace independent users by shared user.
Follow the video in tutorail step 16
www.jms2win.com/tutorial