Because the train doesn´t go all the way to Venice due construction works

Both the Interrail Railplanner + Interrail Website get less updates as the local Railwebsites.

Even the information is incorrect you dont need a reservation (still recommend on the trains to Venice) but you have to pay a Supplement fee of 10€ in 2nd class or 15€ 1st class.

Till end of July the trains will be replaced by buses between Villach and Udine or Venice :)
Between Udine - Venice you can use in that case regionaltrains of Trenitalia.
If you still wanna go by train you can consider to go via Trieste
Vienna - Trieste - Venice or via Innsbruck Vienna - Innsbruck  - (Verona) - Venice

The only direct train currently is the Nightjet that takes a different route via Villa Oppcina (near Trieste)

when and where do i have to pay the supplement fee?

thanks for your reply btw :)

At ANY OeBB counter (then you may even get a free Seat-RES included)-or at the last to conductor on board (it is said there will be a surcharge for doing that-maybe it starts today).

BTW-Vill-Venezia is quite a long ride-a few hrs.

I keep on wondering why it seems so hard for young people to think of the most evident-today I even had to suggest 2 ESpanoles to go to RENFE for their REServ to leave ESpana.