By default bookers can either only book for themselves or other users. It is possible to restrict some users from booking for guest travellers, and/or adding traveller profiles to the team.
If you are unable to add a guest traveller, you would find out when choosing Who is Travelling?, where you may have the option to add a profile.
If you select Add a new profile, you may be presented with one of two screens.
If you have access to book for guest travellers you will see:
A button labelled Add Guest
An optional checkbox labelled Add this traveller to the team.
If do not have access to book for guest travellers you will see:
A button labelled Add Traveller (this profile will be automatically added to the team)
To book for this traveller you can:
Contact your team admin to create a profile for the traveller;
You can share the basket with someone who can book for guest travellers.
You could contact your team admin for access to this permission.
(If you do not have the option to add a profile, you could also share your basket or contact your team admin, to assist you with this)
To share your basket with someone that can book for guest travellers please see this article