How can i Add one more subscription link in course details page

Hi, 1.I want to add one more subscription link in course details page for new user subscription please help me how can i add that link ,i know the hook location but i don't know the location of subscription link where it can be located? 2.when login i'm getting below error please help me.after login the url like this http://192.168.1.98/testsite/user/login EntityMetadataWrapperException: Unknown data property og_user_node. in EntityStructureWrapper->getPropertyInfo() (line 335 of /var/www/html/test/testsite/profiles/opigno_lms/modules/contrib/entity/includes/entity.wrapper.inc). I hope you get quick replay. Thanks in Advance.