sharing files on iCloud
The following error message appears when trying to share a file with another iCloud user:
Your request couldn't be completed. This account has restrictions on the people
you can share files with.
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iCloud: Share files in iCloud Drive on iCloud.com
https://support.apple.com/kb/PH27920?locale=en_US&viewlocale=en_US
Maybe this? "You must be the owner of a file to manage participants or sharing options."