I have to bootable SSD's in my Mac Pro. One with El Capitan and one with Sierra. I wanted to change my startup disk in Sierra to El Capitan but the computer always reboots in Sierra. When I lock the startup disk control panel I get the message that I have unsufficient privileges and I can't change the startup disk to the selected disk.
I have made a screenshot but I can't post it here with the method that I always used in the past. Strange.