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Google released Google Drive File Stream for Mac, and this is what you get when trying to copy a folder from it.

Great.

Now, why isn't copying file a supported operation? And what do I do about it? Are my files being held hostage?

UPD: turns out this error occurs only when you copy a folder that contains those weird Google Docs/Spreadsheets files that aren't really files, but hyperlinks to the web-files.

So, Google introduced some bizzare non-files, then broke the normal filesystem workflow.