Most REServ can only be done 3 or even 2 month advance anyway. As was the universal EUR train system till just a short time ago. And no, really, those trains are not fully booked then the day after opening.
The major hickup in your plans are likely-as always, TrenIt. The rest should be available, best general site is bahn.de. The app of EUrail is not updated enough times.
IF online REServ is possible, then best done via the site of railway concerned-details often in country-info. IN DE=germany you can, but do not need to reserve.
The direct nighttrain STO-Narvik seems to form a problem due to its new operator, VY.se
Trains In NO best done via entur, a neutral general NOrwegian organiser now that the old state NSB is no more. For beds in sleeper they still have special covid-rulings. But for an ITalian that should not be overly surprising.
NOTE-spec for Scandinavia: 21/6, midsommernatt, the longest day (VERY long up north there!!) is THE big holiday and for many a day off-services may be changed, like they are also on Xmas day. Just be aware of it-as ITalians think holidays start only in august
HI; thank you.
Our travel will be from Milan via Amburg - Copenagen - stockholm - oslo - bodoe.
in effect the only train that is not present on the website of the interrail is the one from Milan to Hamburg (exchange in basel). So how can i reserve that train? I do not find any other way.
Interrail can do Norwegian reservations but only on paper and with 9€ postage charges on top of the usual extra booking fee of 2€ per seat.
Norwegian trains are best booked by phone via Entur, details on this page (scroll down to Norway).
Reservations for day trains in Italy and from Italy to Switzerland are best booked via ÖBB using these steps.
Reservations in Germany are best booked via DB (click "Seat only”).
Reservations from Hamburg to Copenhagen are best booked via DSB.
Reservations in Sweden (also the Copenhagen - Stockholm trains) are best booked via SJ (pass cover number needed, if you have a mobile pass, request one here).