You have the mobile pass?You book on b-rail?

In that case-since1/7 it is now possible to generate the cvr nr via the app. Before one had to ask for it. it starts with I (nterrail) or E (urail) then 8 digits.

I´ve seen some report however that E is not accepted somehow-replace with I.

If this does not help-state exactly for what train/from where to where. Or find seewulf general overveiw on howtodo REServ by yourself  and see if that helps

No I have paper pass

@Diego Fernando Morales 

Where are you trying to make the reservation?

Eurail.com

Where did you buy your pass? 

Does your pass cover number start with an E and have 8 digits?

There are several ways to make reservations and the advice from the experienced travellers is to use other ways than the Eurail website. Please read more about that below. 

Here is some useful information from the experienced travellers in the Community regarding both planning and reservations. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations