10.6.7 snow leopard

Trying to upgrade Snow Leopard (10.6.8) to El Capitan, I can not select the primary boot disk called "Snow Leopard" because it is grayed out. When I click on it, I get the following popup message (free translation from Greeks) "The selected disk can not be used as a boot disk". I can confirm...

i couldn't find any guides for this situation i tried multiple times myself to do this and frustrations are mounting up. I have a Gigabyte MotherBoard Z68XP UD5, CPU Intel 2700K SandyBridge, AMD Radeon 6870 HD Graphics card and have fully running Snow Leopard 10.6.8 on a SSD which i use for...