I am trying to Install Sierra on a partition of the internal drive of my 2011 iMac 2.8 GHz (Intel Core i7 + 20gb RAM). When I try to install Sierra from the installer I cannot select the targeted partition to complete the installation and receive the following error message. I am using Lion OS 10.7.5 on the other partition so I can run older apps but I would like to dual boot with a newer OS. My research has not revealed a definitive answer