Here at MPN, we do a lot of support work and use project sites and issues to track that work for our clients. When we upgraded from PS2010 to PS2013, some of us were unable to link issues to tasks. When attempting to do so, we got this error message “Sorry this site hasn’t been shared with you”. This message would appear despite the fact that I was the owner of the project and had full access to the project site. Thanks to our support Guru, Kiran Khanvilkar, we were able to resolve the issue by unlinking the Project Site and then linking it back.