I'm stuck, why can't I open the next skill?

You can "unlock" the next row of skills in your course when you've reached level 1 in every lesson up to and including your current skill. 

Once you reach that level 1, you can continue in that current skill or start level 0 in that unlocked skill.  All unlocked skills will appear as purple.

     

Open an unlocked skill to refresh your learned lessons and complete the skill by passing all of its lessons.