So if you removed everyone else it showed, it still didn't let you move forward? If that's the case then the issue is likely the system believes your AP does not meet the eligibility requirements. Everything indicates you can only have one guest and both parties must have passes that are activated as of today and are eligible for admission to the park on the date of the preview.
For example in my case, my currently active AP expires on 9/8, but Ive already renewed it for another year (thru 2022) and it allowed me to book my preview on the new AP that isn't yet in effect until my current one expires.