I should clarify that these are the steps I followed: 1) Configure the event in YouTube Studio (this usually happens 1 week before the event). 2) OBS Manage Broadcast; 3) Select predefined broadcast and choose the event defined in step 1; 4) Select Broadcast and start streaming; 5) Wait for YouTube Studio to show Preview of Broadcast with Excellent Data; 6) Press OBS Go Live. This is when I get the error. 6) Press Go Live in YouTube Studio - and it goes live.
We do not want to use the Auto-Start feature in YouTube as we want to preview the event and confirm sound quality etc before Golng Live.