According to Etihad Policy Hub
https://www.etihadhub.com/en-ae/policy-library When the surname is not available, update the name using one of the following formats:
Enter the given name in the last name field and enter the given name as FNU (First Name Unknown) followed by the title. LNU must not be used in the name field. Update the name on the booking as HABEEB MOHAMMAD/FNU MR
Enter the given name(s) in the last name field and add a title in the first name field is accepted by Etihad Airways. E.g. HABEEB MOHAMMED/MR