Thanks for reaching out. We are sorry for the trouble with Lightroom. We'll need more information to help you with this.
Could you please share the version of Lightroom Desktop, Lightroom Mobile (Android) & the operating system you are working on? Does this happen with all the videos from the Pixel Device, or does it happen with specific files?
Have you tried this workflow with any other generic MP4 media files?
Would you mind sharing a sample video from your Pixel device that you cannot export on Lightroom Desktop?
Try this
Create a test album in Lightroom with the videos you are trying to export.
Right-click on the album and save the album locally.
Export the album to an internal storage location.
Also, please log on to Lightroom Web (https://lightroom.adobe.com) to check current Sync issues on the left part of the screen.
If there are any sync issues, click on them to know more, find the original for those images' original or source location, let them finish syncing, and back them up safely before you empty the Sync issues from Lightroom Web.
As long as there are any sync issues on Lightroom Web, you can have difficulties exporting & syncing edits & images & videos in the Lightroom Ecosystem.
Hah, got it. If I export the *original* file, it works fine. If I try export after adjusting Exposure or Contrast, it blows up with that "'develop' field" error. I can attach the original mp4 if you'd like.
By the way, I dropped by Lightroom Web as you suggested but the screen I see looks nothing like your screenshot; I suspect others might have the same experience? In any case no sync errors are reported and in the Lr app it says everything is fully synced.