Trial session are not free. There are very cheap but still you have to pay for it. For pay you have to put money (credit) on you account. After, you go on a teacher profil and book a trial session

Most teachers charge a very low rate for trial lessons but not for free, because often you wait for someone and they don't come, so it is still your time. After the trial lesson you decide if you enjoyed the lesson and you want to continue or not.