Hey there VKimble,
Thanks for reaching out to Apple Support Communities. I understand you’re having issues downloading iMovie to your Mac. I’ll be happy to help with this.
If you haven’t already, restart your Mac and try downloading once more:
How to turn your Mac on or off
You may also want to check your Mac storage just to be sure you have plenty of space:
How to free up storage space on your Mac
If that doesn’t help, see if this occurs in safe boot or a new user account to isolate and possibly resolve this issue:
Use safe mode to isolate issues with your Mac
How to test an issue in another user account on your Mac
Let me know if this helps.