Folder Encryption

If you have an admin account and create an empty folder ( right click on the desktop and click on new folder )

Open finder under iCloud Drive > desktop you can see untitled folder , select it and right click on it and click on get info .

Open the small golden lock in the bottom right by entering the admin name and password and check the box of locked and then close the small golden lock and close the window .

This locked folder will be created in the finder with your admin password only , you can give privileges to this folder .

See this article macOS Sierra: Set permissions for items on your Mac

You are partially incorrect. Although you can't technically encrypt or protect a folder without a passcode you can do it with a disk image using disk utility. https://support.apple.com/guide/disk-utility/create-a-disk-image-dskutl11888/mac

Follow that link. If you still need help reply back.

Yes, but not using macOS.

Is this answer posted by Kappy as - Yes, but not using macOS.

solves the question technically , very strange .