Compressor missing/unfixable - cannot export projects from iMovie

I have a 2016 Macbook Pro running OS 10.12.6 and iMovie 10.1.8.

I am trying to export projects from iMovie. I could do it a week ago (to file and to YouTube), but now even the projects that worked a week ago are not working now. The outcome I want is to be able to export projects to file and YouTube.

When I try to export the video anywhere, I get the following error message immediately on trying to export a project: "The operation couldn’t be completed. (com.apple.Compressor.CompressorKit.ErrorDomain error -1.)"

I am aware from searching and troubleshooting that this is a common error with no single fix. I am hoping someone can see through the weeds for me and see something I have missed. I have tried the following, based on suggestions in previous threads:

• Reboot in safe mode, immediately followed by normal mode.
• Updating iMovie. (It will only go to 10.1.8 because I am using OS 10.12.6. I cannot upgrade to Catalina yet because Catalina makes many of the apps I need obsolete. Let me know if this is likely to solve the problem though.)
• Looking for the Compressor folder in Library>Application Support. It is not there.
• Looking for the Compressor by right clicking on the application icon for iMovie. I tried to move that folder (under plugins) to a second location, but it would only copy and move, not move outright.
• Trying to access iMovie from a different user account. I could not do this because the only other user account available to me is a guest account with safari only, which, when I accessed it, would not respond to my keyboard inputs or clicking on anything. It had no apps other than Safari.
• I tried to read the Apple user manual for the Compressor but I couldn't understand it or how it pertained to iMovie, or why it would suddenly have changed. It also didn't have any trouble shooting information as far as I could see.
• I googled the Compressor in general and found people talking about it as if it were an app, with different versions, that you can buy or download from the App store. I don't know if that is the same thing as my issue and when I search the Compressor in the app store only 3rd party apps come up. I had never previously downloaded a compressor app; I was just using iMovie as it presented to me when I got my computer.

I have been using Apple products for a few years now. I'm good with all the everyday stuff and fluent in using most of Apple's apps, but I know next to nothing about how the OS works or anything like that. (Finding the library folder was a stretch, for example.)

If this isn't something someone here can help with, do you think it's something a tech support person could help with? I don't have access to Apple support where I am but am hoping it's some no-brainer fix for someone who works on this stuff all the time.

Many thanks in advance for your help.