Which Route?

Maybe Eurail cant reserve the Route. Or you forgot to add some details in the travler information it´s difficult to say without a Screenshot. Usually when you can´t proceed it shows at the top what the problem is.

Utrecht to copenhagen

copenhagen to lugano

milan to padova

padova to klagenfurt

klagenfurt to wien

at the top it doesnt say anything, the last button proceed to payment just is not clickable

Utrecht  - Hamburg can be done on bahn.de “Seat only”
Hamburg - Copenhagen depends on the train for a daytrain
https://travel.b-europe.com/dsb-rail/en/reservation-only#TravelWish

Copenhagen - Hamburg same link as above
Hamburg - Lugano can be reserved via bahn.de “Seat only” if you wanna use the daytrains.
If you wanna get a bed in sleeper or bunk in couchette between Hamburg - Zürich you have to reserve via a special way on oebb.at :) 

Milan - Padova can be done with a change in Verona reservation free
or you take the Highspeed train and reserve via oebb.at by following these steps

Padova - Klagenfurt
depends which trains you take i recommend to catch a regional train to Venice and from Venice to Klagenfurt the Railjet (which date are you traveling ? as this summer there is a busreplacement between Venice - Villach on the route to Klagenfurt )
Here the link how to reserve Venice - Klagenfurt
 

and finally Klagenfurt - Vienna can be reserved bestway via oebb.at
 

I recommend when ever possible to reserve directly with the railcompanies when ever possible as this is the cheaper and in my view easier as with Eurail/Interrail


The links are the same Topic but always to the relevant post in the topics with manuals how to reserve.