the short answer
A money-back guarantee is risk reversal: you remove the buyer's fear so more people say yes. The safest version for most coaches is effort-based - do the agreed work for a set window, see no change, and get a refund - because it rewards the one thing you actually influence. Avoid guaranteeing a specific body or health result; you cannot control adherence, genetics, or life, and an outcome promise you cannot keep can stray into deceptive marketing. Whatever you offer, define the terms in writing and route payments through your own processor so refunds stay in your hands.
This article is general information for coaches, not legal advice. Consumer-protection and advertising rules vary by country and state, so confirm any guarantee wording with a qualified professional before you publish it.