If you updated FCP to version 10.4.9/10.5 and your viewport shows a red screen with a yellow exclamation mark (which is a Missing Media type of alert), then you've most likely encountered an old Motion/FCP bug, that has reappeared recently: in the past, the bug sometimes appeared after installing any apps/plugins/updates but nowadays it seems to occur if your Motion or FCP was not installed directly from the App Store.
To check whether your system is affected by this issue, try using some of the elements that are built into FCP:
Titles > Lower Thirds > Date/Time
Generators > Backgrounds > Clouds
Effects > Blur > Directional
Transitions > Lights > Bloom
► If any of these elements display a red Missing Media type alert, then you've encountered the aforementioned bug.
So far the only solution that we are aware of is re-installing the host app directly via the App Store.
To do so please go to your Applications folder, delete Final Cut Pro.app and/or Motion.app, and empty your Bin (important!), then use App Store to re-install host apps.
► If built-in templates work correctly, then please try reinstalling affected plugins using mInstaller and see if the problem persists. If so, try using the manual PKG installers that can be downloaded from your 'My Products' page (the download link will appear once you hover your cursor over the preview miniature): https://www.motionvfx.com/user/products
Be sure to report this bug to Apple via their feedback page:
Final Cur Pro: https://www.apple.com/feedback/finalcutpro.html
The problem is very widespread, so the more reports are sent, the higher the chance that it will get fixed sometime soon.
If you see the plugin registration dialog upon video export, see THIS page.