You are combining audio and video clips outside the primary storyline.

Others have posted on this board with the same problem. I don't know what could be causing it. iMovie does not have a compound clips feature, unlike Final Cut Pro. You might try contacting Apple Support by clicking on the Support item in the upper right hand corner of this forum's screen and navigating to a place where you can enter your phone number. It might be that you will need to upgrade your operating system, but I don't know. Deleting preferences and/or redownloading iMovie has not worked for others.

-- Rich

Thanks, running Ventura 13.2.1 so i don't think that's the problem. I was able to power through to get it to work, not sure exactly how, clicking "undo" a lot until something appeared back on the timeline. Once that happened I started again as if those clips were no longer there, and once I had the project going I went back to delete again what I didn't want. Still having loads of other issues, but I'm assuming they are user error not a glitch which I think this is.

Congrats on getting it to work. I haven’t heard of anyone else that has worked through this error.

— Rich