Try to use the Pass Cover Number you have and see if it works.

Otherwise you could 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.

To get the Pass Cover Number for your mobile pass you need to contact Customer Support through this form and asked for it.

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.

Unfortunately, SNCF refused to give me the ticket saying they only accept French, German or American express cards. I have an Italian card associated with the NEXI (mastercard) circuit and they unkindly hung up the phone. I don't understand how a foreigner can book a ticket for a French train if they refuse any kind of sale. I wrote to Interrail support to get my cover pass number and I am awaiting an answer.

Did you try to use your old pass cover number at b-europe?

I entered it and it takes me to the payment form. Can I proceed with the payment or is it preferable to wait for the assistance response?

The response from Customer Support will probably take many days so I would say that you proceed with the payment.

They are lying to you. Over the past few months, we have seen multiple reports of people who could book via SNCF, from various countries.

Or they very recently changed something and are now breaching the EU SEPA regulation.

The pass cover number is not printed on the reservations soi just go ahead.