The "outside the primary storyline" message is pesky. Others have complained about it. Also, iMovie does not have a "compound clips" feature like Final Cut Pro has, so the message is indeed curious. Take a look at this thread for a poster who resolved the issue by clicking on "Edit/Undo" a number of times.
You are combining audio and video clips o… - Apple Community
Also, you might try rebooting and starting a new project where there is no "story line" as yet.
Another thing you can try is deleting/resetting preferences. To delete/reset preferences open iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Re-open your original library to get back to your projects. Deleting preferences is a safe procedure and will not cause loss of data or disruption to your projects.
-- Rich