Zoom Webinar links not working

I set up a webinar with additional links to panelists - all inside our organization. When we started a tech check 30 minutes prior to the webinar, some were able to join, and some could not and received this message:

"The host requires authentication on the commercial Zoom account platform to join this meeting. Please sign in with a commercial Zoom account to join."

I did not require any special authentication or password when the webinar was set up. Links on all previous meetings and webinars worked without any problems.

We were supposed to host a webinar for about 500 people and had to switch to Teams at the last minute since we were not sure how many people might experience this problem.

Any idea what went wrong?

thank you!