You need to contacted Customer Support through this form and asked 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, here you can get advice on alternative ways to make your reservation. 

Traveling with my with my wife from PARIS TO NICE on 25/06/2022.

i have eurail mobile pass.

need two reservation ticket

train time - 09:07 - 15:07

The best way to make the reservation is 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.