We need a few more details to advise you.
What are you trying to book and when?

Who are you trying to make the reservation with?

I’m trying to book a travel from Copenhagen to Bruxelles-Midi on 15 April 7:07 from Copenhagen. This travel have three legs, Copenhagen to Hamburg, Hamburg to Koeln and Koeln to Bruxelles. I’m trying to book this journey for me and my wife in 1st class.

Try to reserve CPH-Hamburg via this website https://travel.b-europe.com/dsb-rail/en/reservation-only#TravelWish

And Hamburg-Brussels via the seat only option on https://www.bahn.com/en.

It seems that there’s still availability for the most crucial train (CPH-Hamburg) and plenty of availability on the German trains too.

Try the suggestion above. I normally use the DB website (direct url to seat reservation: https://reiseauskunft.bahn.de/bin/query.exe/en?&rit=yes) 

It should work. Enjoy your trip. 

That seems to work - thanks!

But I still don’t understand the error message from Interrail….

The Interrail reservation service is way too expensive for this journey (€8 per person and train). First try to get reservations for the whole journey via bahn.com, see link above (€4.50 per person and journey).

I suspect very few of us understand many of the messages from the Interrail reservation system, including this one.

I can only imagine somewhere there is or was a promotion for free reservations and somehow the system has this message still in its memory banks. I have not heard of this one before.