You may have opened the carnet using a shared link.

Shared links provide read-only access and are designed to allow users to view carnet information and present existing transaction codes where appropriate. They do not provide full carnet-management rights.

How do I gain full access?

To prepare travels, edit carnet data where permitted, and manage the carnet fully, you must download the carnet using the Carnet ID and PIN provided by the Issuing Association.

Why is the "Prepare Travel" button unavailable?

If the carnet was accessed through a shared link, travel preparation functions are intentionally disabled because the user has read-only access.