Assign recording privileges to attendees is grayed out in Webex Events on Windows
Record the event checkbox is disabled on the Attendee Privileges screen in Webex Events
MP4 Recordings Default in Webex Meetings 40.10
In the upcoming October (40.10) update, all-new recordings in Webex Meetings will be stored in MP4 format, either in the cloud or locally as selected at the site or host level, with a video-centric experience. By standardizing the recording format, you'll have a wider choice of playback tools, better security, and a more effortless collaboration experience even after your meetings. Existing ARF and WRF recordings can still be downloaded or played at the Webex site. For more information regarding video-centric recording, go to Video-Centric Network-Based MP4 Recordings in Webex Meetings and Webex Events.
This is a known issue with Webex Events on the Windows client, and this option will be removed as soon as .WRF reaches end of life.