You're not the only one with problems. You didn't say which site you try to make a reservation on but I guess it's the Interrail website.

If you say which trains you want to book, then people can advise other places to book.

Hi, When creating a trip, you'll need to add travellers to that trip. When making reservations, you are required to choose a trip, and the travellers connected to the trip will then appear in the timetable section. You can then select them. 

If none appear, please check the my trips & travellers section in your account, by clicking on your name at the top right. From there you can edit your trips and and add travellers if necessary.Cheers,