Final Cut Pro X Exporting Problem

It appears you are exporting to an AVI file. Is that correct? That should work but I'd suggest you try exporting to 1080p H.264 like this:

Share>Master File>Settings, then pick:
Format: Computer
Video Codec: H.264 Better Quality

In most cases you can pick H.264 "Faster Encode" and there will be a visible quality difference, but it will export faster.

If this doesn't work, state what your original camera format is and whether you imported with "leave files in place" and whether you created optimized media.

Also is there any particular reason you are running a beta version of El Capitan? What version of FCPX are you running?

 

Here are some suggestions:

(1) Upgrade both OS X and FCPX to the latest versions. If we spend time chasing a problem that could have been fixed by upgrading, it is all wasted.

(2) After upgrading, delete all your generated library files and re-render the project. This will not affect any edits. Procedure: Select Library in left-hand libraries pane, then File>Delete Generated Library Files, select all checkboxes and delete.

(3) If you were using optimized or proxy media select those clips in the Event Browser (or just select them all), right-click>Transcode Media and pick optimized and/or Proxy.

(4) Select the entire timeline and then Modify>Render All.

(5) If it renders OK in the timeline then export to H264 using the above-listed settings.

Re AVI, this is a container format and does not reveal much about what codec is used. In general FCPX is very good at using camera native format for editing, but since you are having a problem I'd suggest any AVI content be transcoded to optimized media. This is not generally necessary, but just a troubleshooting approach. It can be done after import within FCPX as mentioned above.

If you want to share the final product for web, computer or mobile devices, exporting using H264 using the above-mentioned settings is usually best. That will produce an H264 .mp4 file.

 

The beta definitely seems like one troubleshooting avenue. In my (limited but existent) experience, Apple's apps are sometimes so intermingled with libraries provided in the system that I'd recommend not installing betas on any machine on which you depend on using them.