I am using Mojave and iPhone 6s. I don't use iMovie for iOS much, but tried a test and had no problem exporting an iOS iMovie project to my Mac's downloads folder. So I don't know what is going on with your system.
Since the export fails on the way to the Downloads folder it has not reached the Mac iMovie app yet, so not a Mac iMovie issue. Perhaps the error is coming from your iPhone or some issue with AirDrop. Or maybe there is some corruption in the iOS movie project. The error message does say that the problem is with the "Import", so possibly a glitch in your Mac's system.
Instead of using AirDrop does your iOS project export options include other ways to export? If so, you can try those.
In any event, you might find it beneficial to call Apple Support, as the savvy techs there have the capability to remotely access your Mac, that we can't do here. Click on the Contact Support item in the upper right of this forum's screen. Navigate to a place where you can enter your phone number.
-- Rich