upgrade from Maverics to Sierra/High Sierra

Hi I am having trouble trying to upgrade from Mavericks to Sierra - get an error message about a signature failure. Cannot find an solution. As far as I can tell I need to go Mavericks - Yosemite - Sierra - High Sierra. I have found a link that goes from Snow Leopard to El Capitan to High Sierra. Do I have to regress to go forward? If so how do I regress to Snow Leopard?

Thanks in advance Joanne

iMac Mid 2011, Mavericks 10.9.5