You pass is your ticket. Do you mean you want to book reservations? You can do that anywhere you like. There's no obligation to do that via Interrail. You can also do that at a station or via the operators’ websites.

What do you mean by "doesn't work”? Is there an error message and if so, could you post a screenshot? Or else, please post a screenshot of where you get stuck.

In addition, if you give details of the train you'd like to book (origin, destination, date, departure time), then people can suggest alternative ways to book.