Customer service can get you one.

What train do you want to book?

I wanted to take this one.

I already contacted them but I’m not sure if they will answer in time 

You should be able to book by phone at 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.

I have exactly the same problem, and I’m trying to book train tickets for tomorrow so I really need to get this sorted ASAP. The Interail website has a bug and won’t let me book anything, really regret ever getting an interail ticket and would like to ditch the whole thing and get a refund instead

@Becky Brown 

You can book Eurostar and French trains by phone at 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

Hi, I would like to get a pass cover number for my digital pass too.

Thanks.

My first answer is still valid:

Customer service can get you one.

I also need a pass cover number. Submitted a ticket to customer service but not sure I'll get it in time

@Jason Eric Gordon 

What route do you want to make a booking for? Reservations can be made in several ways.

The interrail.eu site did not work (just kept returning to teh Home Page), and the b-Europe site asked for a pass number that wasn’t acceptable because we only have a digital one.

We then tried the Eurail.com site and this did work. The site is a mirror of the interrail one but you pay in USD or Eur instead of £’s, which makes it slightly cheaper with exchange rate.

This might be useful information for those having problems booking Interrail reservations on Eurostar..