Klaviyo Lists & Segments - Opt-in Process controls how Sprwt adds people to your Klaviyo lists during the automatic list sync:

• Single opt-in (default): customers are added directly as list members and removed directly when they no longer qualify.
• Double opt-in: customers are added as subscribers, so Klaviyo sends them a confirmation email and only adds them once they confirm; removals are done as unsubscribes.

Pick the option that matches the list settings inside your Klaviyo account - if the Klaviyo list itself is double opt-in, people will not appear until they confirm regardless of this setting.