iMovie Error: RequestFrameAt returned: 3 for absolute frame: 0

The error message is saying that there is a corruption at the very beginning of the movie, at frame 0. Possibly it could be a title or black frame, or transition. Try using the slider above and to the right of the timeline to expand out the timeline. See if you can see any signs of corruption at the beginning of the movie, such as an artifact or black frame. Trim out a few frames from the beginning of the movie and see if it will share out without the error message. Remove any title or transition or effect. Then try to share out again. You might want to make a duplicate of your movie and work with the duplicate so as to preserve your original.

-- Rich

If the procedure in my previous post didn't resolve the issue, try doing an Edit/Select All, Edit/Copy, of all the media in the project and then Edit/Paste it into a newly created project. Now see if you can share out without the message error.

Also, are the film clips in the movie possible in a frame rate not supported by iMovie? Or maybe in an HDR or HEVC format?

-- Rich