authentication server failed to complete the requested operation

Hello PatricioVincenzo

Thanks for posting your question in Apple Support Communities.

We understand that your friend is receiving an error message when trying to enable FileVault.

"Some of the formatting information on the disk may need repair. Try using Disk Utility to repair the formatting. Disk Utility examines your disk’s formatting information for errors, then repairs any it finds. See Repair a storage device in Disk Utility."

Click here for more details: If you can’t turn on FileVault on Mac

Best wishes.