You need contacted Customer Support through this form and ask for the Pass Cover Number. 

https://eurail.zendesk.com/hc/en-001/requests/new

Customer Support is currently overloaded with requests so you will probably have to be patient to get help. Please let Customer Support know what date your travel will start so that they can prioritise your request correctly.

For most routes there are different possibilities to make reservations and some don't require the Pass Cover Number.

If you give your travel details: date, departure time and route, in a new post you can get advice on alternative ways to make your reservation.

@AnnaB  Yup thanks i have contacted them, just hoping for one of them to answer me quickly haha. Thank you for your help!