There are a lot of informations missing.
If you did book a direct EC train, you don't need a reservation, you do need a supplement (for every type of pass) for 10 € 2nd class or 15 € 1st class (depending which class your pass is).
If you are holding an Italy pass and not a global pass, you do need an additional ticket for Innsbruck to Brenner.
Whatever you need: Buy it before boarding the train (available online (ÖBB ticket shop) or at the station).
I have eurail pass and train is ECB83
I have eurail pass and train is ECB83
Missing information that makes a difference:
Do you hold an Eurail one country pass for Italy or a global pass?
Do you hold a 1st or 2nd class pass?
What did you already buy for the EC 83 train? Just a seat reservation? How much did you pay?