No. Neither do most other healthcare providers. Your trainer won't, your weight loss advisor won't and so on.
In hypnosis, I play a navigator role. I call out directions and if you want to get where we are going, you follow them. I'm good at giving directions that follow a route that you will approve of. But if you choose not to follow my directions, I simply can't guarantee where you are going to end up.
This doesn't apply so much to session work, done while we are together, but virtually every type of hypnotherapy comes with homework. For example; If I recommend that you keep a regular bedtime to assist with sleep improvement and stress reduction, then you work on your laptop in bed until the wee hours of the morning, I can't promise results.
With that said, I have really good success rates and the odds are that if I accept you as a client, it's because I am confident I can help you meet your goals and I believe you are going to do the work. I generally have a client waiting list and can't take clients I don't expect to succeed.
I WANT you to succeed and I will work with you to make it happen. I will even fire you as a client if I sense that you aren't doing your part. But I can't offer a "money back guarantee" like a retail store.
The one exception would be if for some reason I could not get you to go into trance during our first session. I would, of course, refund your money, since I had not done my part. THAT HAS NOT HAPPENED YET.