When I tried to put my Desktop and Document Folders on iCloud:Your Documents and Desktop folders cannot be stored on iCloud Drive. You have too many files, too many files in a single folder, or too many nested folders in your Documents or Desktop fol

Ran in to the same problem here... signing out of iCloud, restarting the computer and signing in again worked for me.

I have the same problem. Unfortunately, there doesn't seem to be any way to determine the exact problem in order to fix it.

I have 423.6 GB with 1,100,147 items in my Documents folder and 5.37 GB with 210 items in my Desktop folder. I have a 2 TB plan (as of Apple's free upgrade last night!) with only about 538 GB used.

Before I activated the option to sync my Desktop and Documents folder, I moved all files and folders from my Desktop to a file "Desktop Local" in my Home folder, and all files and folders from "Documents" to a similar folder "Documents local".

With the Desktop and Documents folder now being empty, I enabled the sync with iCloud Drive for Desktop and Documents, and it started without problems. Then I selectively moved the folders and documents from "Documents local" to the empty Documents folder. This way I ensured that I did not start to sync all my clutter to iCloud, but only files and folders I really want on all devices.

And I try to keep the Desktop empty, so I can quickly see the files I am dropping there.

If the document folder contains several hundreds GB of files, this is a prudent method to enable sync. During sync, if something strange happens, only a smaller number of files will be affected.

My question is, after syncing the document folder under iCloud Drive to iCloud, is there a way to prevent all the files move to the iCloud that is there are two copies of the same file, one stored in iCloud.com and one store in the iCloud drive's local document folder? The user can access the local copy when there is no internet connection.

If you want a local copy on your hard drive, check System Preferences/iCloud/iCloud Drive/Options and make sure Optimization isn't enabled.