That error message can occur if one or more clips in the project do not reside on your computer, but are still in the camera. They may not have been imported properly, such as from disconnecting the camera before the clips have finished importing. In that case you will need to reimport the clips. Or maybe the camera or memory card is still connected after importing, in which case you should remove the memory card or disconnect the camera.
Do any of the clips in the media browser show camera icons on them? When you select a clip in the project and do File/Reveal in Finder, does the clip appear highlighted in the Original Media folder that pops up? If the clip is absent in the Original Media folder that would mean that the clip is not in the iMovie library.
Here's a link to the online help menu item that explains how to import from a file based camera (that I am assuming that you have).
https://help.apple.com/imovie/mac/10.1/#/movbc953953e
The above link conains a link to Apple's site that lists camera that are supported by iMovie. See if your camera is supported.
-- Rich