Can you state the trains or journeys you want to undertake? And when you would like to travel on each of these journeys? Otherwise it's difficult to help you get to proper seat reservations. 

Reservations are completely separate from the pass, there are many other ways to get them. You don't necessarily need to book via Eurail, nor need you to link that Eurail account to your pass that you bought elsewhere, in order to get seat reservations. 

The topic below has al lot of information, to get your seat reservations. Please read it carefully and feel free to come back to us, if you don't know exactly what to do.

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Yes, you do. You must link your account to a pass in order to be allowed to make reservations. It is only possible to make reservations through the Eurail website if your account is connected to a pass. At least when I tried.

But as I understand, the person above bought a pass via a third party website, and is already using the pass. Is that correct @a.m ? So I actually don't know if it works by linking these passes to an account on Eurail, never tried this myself before

But for most reservations, you can work around the Eurail website. That's what I meant @AnnaB 

As far as I know all passes, no matter where they are bought, can be linked to an Interrail/Eurail account.

To link a mobile pass you need the mobile pass number and the name of the traveller.

To link a paper pass you need some more information, i. e. the issuer of the paper pass, where Eurail only is one on the long list of issuer to choose from.

thank you for answer.

I wanted to go to Bergen from Oslo, Norway. However, I didn't know how to make a reservation online, so I did it locally. And in the future, there will be times when you will be riding trains that require reservations. At that time, I would like to make a reservation online in advance instead of making a reservation at the site just before that time. By the way, I have already used the pass that does not require a reservation from the app without any problems.

Reservations for Norway are best bought locally (but there are only 5 ticket offices) or by phone via Entur (add +47 for Norway). Reservations are then sent via email. I'd avoid booking via Interrail/Eurail because you pay an extra booking fee (€2 per person and train) and a €9 postage fee because they only issue them on paper.

If you tell us which other travels you have planned to take, we can tell you the following:

• Where and how you can get reservations online, other than via Eurail.
• If the train has mandatory, optional or no seat reservations
• Possible alternative routes for those trains with (sometimes expensive) mandatory reservations.

I am using a mobile pass. 

I don’t know any postage service.

I have more trips planned than I can write here. So here are some examples.
・Bergen → Voss (to go from Bergen to Sognefjord)
・ Stockholm → Abisko sleeper train
・ Rovaniemi → Finland Santa Claus Express

Whether you have a mobile or a paper pass is not relevant for the way to make reservations and the type of reservations available (on paper or on PDF).

Book via Entur, either by phone or at a ticket office.

Book on sj.se/en

Book via VR, either at a ticket office or by phone:

https://www.vr.fi/en/customer-service

Sorry for late reply.

I was able to reserve successfully.

Thanks!