The problems that you describe can be caused by a number of things. Does the "Export Fails" message contain any more information? If so, please post its exact wording.
Try working your way through these troubleshooting procedures to see if you can isolate the problem:
Deleting Preferences
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 library. Reopen
your old library to get back to your projects. Deleting preferences is a safe procedure that
will not cause data loss or disruption to your project.
Opening iMovie in a new library
Try opening iMovie in a new library. To do
that, open iMovie while holding down the Option key and then selecting to
create a new library in the box that appears.
Opening iMovie in a new user account
Create a new user account from System Preferences/Users and Groups and try to open and
use iMovie in the new account. You can use the Guest account on your Mac for
this purpose.
New Project
Another thing you can try is to copy your media into a newly created project and see if
that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the
media in your project and then Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and,
after everything loads, immediately shut down and reboot in normal mode. Then
see if iMovie works properly.
Updating iMovie
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications
folder into the trash, but do not empty the trash. Sign in to the app
store and open your purchases folder by clicking on your name at the bottom of
the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update
iMovie from there. If not successful, drag your present iMovie app out of the
trash and return it to the Applications folder.
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new
M1 computers):
https://support.apple.com/en-us/HT201295
Resetting the NVRAM
or PRAM
Try resetting the NVRAM or PRAM, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
Contact Apple Support
If none of the above resolve your issue, contact Apple Support. The techs there have the capability of
remotely accessing your computer that we cannot do here. To contact apple support click on the Contact
Support item in the upper right hand corner of this forum’s screen. Navigate to a place where you can enter your
phone number.
-- Rich