List Action – Add or remove user from a list. Choose an Action (Add user to list or Remove user from list) and a List (one of your active Lists). It's meant to keep lists in sync with behaviour, e.g. add every new registrant to a Welcome list or remove someone from a Prospects list after their first order. Validation: Please select an action (Add or Remove). and Please select a list.

In the current version this block can be configured and saved in the builder, but the background process that walks a flow's steps and queues Email/SMS/Delay/Conditional blocks has no handling at all for List Action steps – when a contact's journey reaches one, it's silently skipped (no list add/remove happens) and the journey simply continues to whatever block comes after it. Treat this block as not yet functional and update list membership another way (manually, or through a Conditional plus a separate campaign) until it's completed.