Actually, I am Tech Support Engineer.
Is your profile local o remote? (where is your documents folder located?)
Check NTFS permissions on your disk, you should have full control.
Check disk health with an appropiate software
Is the disk already encrypted with another software?
Close every security software and try again,
Also try encrypting in safe mode.
If no luck with these steps, you will have to create a new case to Symantec support so we can troubleshoot in deep.
Regards