Open the tree. And after?
Tomorrow i will finish the tree. I have level 14, fluency 56%. After I will have finished the tree what I will do? The final level is 25. Must buy courses?
Have you reviewed? The final level is 25 because reviewing will help you internalize the words and structures much better than just getting to level 14. You don't have to buy anything. There are plenty of free things you can use online to learn more. Memrise is another good website. That will help with vocabulary. Reading and listening are key to getting better. Even watching something with subtitles in your mother tongue is helpful if you do it frequently.
No, you cannot buy any additional lessons.
When you have finished the tree, you have seen everything that Duolingo teaches.
If you go back and repeat lessons to practice words and grammar, you will gain more XP (experience points) and then your level number will go up. But there will be nothing new on Duolingo.
It is perhaps time to look for other resources as well.