Hello @JenT90 ,
It looks like this is an issue our engineers are investigating, which stems from a very specific scenario involving RSVP, Poll, and Eventbrite blocks. The issue occurs when the top-most checkmark box is used to select-all the contacts, but the page view is less than the full select-all. The issue has been tracked to your account, and submitted to the engineers' queue. If and when there's an update to the issue, they'll notify you directly.
To get around the issue you're running into, you can increase the results per page view to a higher number (e.g. 25, 50, etc.) From there the select-all and Add to List will allow the pop-up window to appear, and allow for the standard add process.