What route are you trying to book? There are several ways to make reservations and most routes can be booked without Pass Cover Number. 

various routes, París to Brussels but not available on interrail so trying to book on be-Europe. Would be easier to just have it. Thanks 

You can call  b-europe and make the reservation. No need of Pass Cover Number.

The phone call is rather expensive.

https://www.b-europe.com/EN/Contact/Contact-Centre

The best way to make reservations for French trains is to call SNCF. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail

To get your Pass Cover Number, if you still will need it, you need to contact 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.