Joining clips to make Imovie on MAC

1. The small spaces between some clips (scenes) are called "Gaps." They happen sometimes when you are editing and don't butt clips right against each other. And it can happen if you delete a clip or transition and inadvertently leave a gap where the deleted portion was previously located.) There is an easy way to eliminate the gaps. Watch this short video that explains the process.
2. Regarding the export, you probably had selected only the first clip (intentionally or accidentally). Either make sure no clip is selected; or select all clips before exporting.

You don't need to join the clips. If iMovie is working properly it will export the entire movie.

However, possibly your destination for the video has insufficient free space, or your internal drive

has insufficient free space to allow iMovie to function properly,

Another possibility is that a preference has gone awry. Try deleting/resetting preferences. To do that, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes. Now see if you can get a complete export.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

-- Rich

Just drag them to the timeline and butt them up next to each other.

Once they are all on the timeline, just export or "share" your movie.