I have leveled up and see no way to access level 12. It is not offering me work to do
You get to the next level by doing new lessons or practicing skills you have already done. The tree is what people on Duolingo call all the skills combined. When you do each skill and go down the skills, think of it as descending down a tree. Or maybe climbing up it:) If you have more questions don't be afraid to ask:)
Your level does not directly reflect how much of the tree you have done, but instead how many xp points you have earned. You earn xp not only by doing lessons the first time, but also by reviewing them and by working on immersion.
I suggest a combination of immersion and reviewing skills you have already done. When you have earned enough additional points, you will go up to level 13 and, eventually, to other levels (with the highest possible level being 25).