Sometimes, it just isn’t a thing.
A Service in the domain is a stateless operation that fulfills a domain-specific task. Often the best indication that you should create a Service in the domain model is when the operation you need to perform feels out of place as a method on an Aggregate (10) or a Value Object (6). To alleviate that uncomfortable feeling, our natural tendency might be to create a static method on the class of an Aggregate Root. However, when using DDD, that tactic is a code smell that likely indicates you need a Service instead.