Also when I book them for just myself it says “High Availability” beneath it. So I really don’t understand why it says “No Availability” when I want to book it for us both/my friend.

There's a long-standing problem in Interrail's booking system. However, you can also book elsewhere and save money at the same time (interrail charges an extra fee of 2 EUR per seat).

1. First, request a pass cover number (you will need it for some of the websites below).
2. You can book trains from the Netherlands to Germany on the DB website; use the "Seat only”/"Nur Sitzplatz” button. Also book all train in one go because with DB you only pay once per journey, which can be for multiple trains.
3. You can book the train from Hamburg to Copenhagen on the DSB website.
4. For trains in Sweden, it's best to book via SJ.

Hi Sanne, this is an issue on our side. As a workaround you will need to go to your account → trips & travellers, and add the missing information. You will be asked to fill out a Pass Cover number, which can be obtained by clicking on the green button '’request Pass Cover number'’ on this page. 

Alternatively, you can book on the websites of the train carriers itself, as suggested by rvdborgt. Cheers, 

Hello I am trying to book seat reservations.  In the checkout I am unable to proceed to Traveler Details although the button is available and clickable.  I have all fields filled out and shipping selected.

It's usually best to book elsewhere. Please have a look at the advice on reservations. If that doesn't work for you, then please open a new topic and mention route, date and departure time.