No need to worry! It’s no problem if there are poses that you can’t fully extend into. The teacher will offer different variations so that everyone can benefit from the postures. Each time you come to class, you will learn a little bit more, practice moving with your breath a little bit more, and most likely, you will be able to achieve the pose you are working towards at some point. Having said that, remember that the flexible yogis you see in pictures did not achieve those poses overnight. Be patient, and give yourself time, and see how the impossible slowly becomes doable.