Source Alpha not working

Hi. 

I was trying to recreate this texture from this tutorial, but there is a part at 4:13, when they used Alpha Source Mode, but that function is disabled for me. What can I do?
Thanks!
https://www.youtube.com/watch?v=PBkFaeKzhGE&t=298s

That function is enabled when it's not connected to other nodes. Once I add a connection that option is disabled. In the attachment you can see both.