I\'ve looked at all the previous questions and no one seems to have a problem as simple as mine. Also I\'ve searched the web and can\'t find a solution.
I\'m new to
In chapter Starting a New Project, you were asked to call your project light. It seems to me that you didn't follow that step correctly and name your project alt_ex_1. That's why you're getting 12007 error, since the compiler has no idea what is the top-level entity in you design.
To solve that problem you can:
Assignments -> Device -> General.Project Navigator (Files -> Set as top-level entity).Btw 1, 2, 3, ... - all about the same problem.