Hi there jkilko,
Thanks for taking the time to join us in Apple Support Communities! We understand you're seeing an error when attempting to upload files to iCloud from your Windows PC. We're glad you're here and we can provide some guidance.
"If you receive an error when you try to move a file from iCloud for Windows, you might need to download the file to your Windows PC. After you download the file, try moving it again. " Have a look at the 'See your files and folders with iCloud Drive' section of this support article for more information:
Set up and use iCloud for Windows
Are you using iCloud Drive with iCloud for Windows? You'll want to be sure you're following the steps in the link below:
View, add, or remove iCloud Drive files in iCloud for Windows
We hope this information is helpful. Don’t hesitate to let us know if you have any more questions.
Take care!