Is input field validation possible?

This is possible in the context of a Journey when the mistake is detectable on the target application (for example, by showing an error message). In this context, a Journey step will be visible (or skipped) if there is a given value in the form (contains/doesn’t contain/equals/ doesn’t equal). E.g. on the Journey step, you have entered an invoice number without a hyphen, which means it is invalid. You can define a Journey Skippable Step which will appear only when the user makes a mistake and then define a Step Action to retake the user through the Step where they made a mistake.

For more details on how to configure this, please check this article.