Are you on interrail.eu? Sounds like you're not.

By the way, you can also buy this pass from DSB, at the station in Aarhus, Odense or Copenhagen or by phone:

https://www.dsb.dk/find-produkter-og-services/interrail2/

That may be cheaper and faster than ordering it via Interrail, since this pass only exists as a paper pass.

Hej, tak for svar. Taler du dansk?

My Danish is very limited :)

The ferry service provider may have specific restrictions or limitations on the countries they cater to or accept as the country of residence. It's possible that they do not currently support Denmark as an option for country of residence in their online booking system.