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
You can book €*-assuming there is still quota for passholders on your date/time online via b-rail without covernr, for an extra 4€-which saves the cost for calling them and waiting on hold on a foreign nr. JUst use google for quick landing.
You can book €*-assuming there is still quota for passholders on your date/time online via b-rail without covernr, for an extra 4€-which saves the cost for calling them and waiting on hold on a foreign nr. JUst use google for quick landing.
Calling b-europe from UK is a foreign nr and b-europe charges an extra fee of 0.3 EUR per minute so calling SNCF is cheaper.
@eliwilson112 What route are you trying to book?