If you do not have a user's details but wish to make a booking on their behalf, select the User booking option and under Holder select 'Casual user (no details needed)'.
This is also the option that is selected by default.
This will allow you to make a booking without user details, but you can still allocate a price to the booking. Keep in mind that it is not possible to change the owner of a booking, or to add the user's details at a later stage. You could add information to the title and notes sections of the booking.