Pass cover number for mobile pass

Hi, 

I’m trying to book 4 seats for my family in https://travel.b-europe.com/, bc https://www.eurail.com/en doesn’t let me, and it’s asking for a Pass Cover Number, which i don’t have. I’ve already tried all the different possible numbers but i’m still getting “You have entered an invalid passholder number”. Please can someone help me? I’m traveling in 2 weeks!

Thanks!