You should have received an email with steps to add your pass in to app and how to activate the pass. Did you follow these steps? Did you check the mobile pass FAQ?
The pass must be activated before boarding the first train and any train you use must be added to the pass too. Seat reservations should be bought before boarding a train; some (like for Eurostar) have limited availability for pass users so it's best to book these in advance. Many trains have optional reservations or none at all, depending on country and operator.
For covid requirements, please check reopen.europa.eu. Be warned though that these change often and with little notice.