You want to travel Innsbruck to Verona at 1524, right?

I don’t know why it’s not showing in the Rail Planner, but you can add the journey manually at the bottom of that page.

You also need to pay a €10 supplement to use this train. Search on ÖBB again, and add your railpass, then search ‘one way tickets and day tickets’. This allows you to use the train, but doesn’t guarantee you a seat. You can also add a seat reservation if you want.

So, am I understanding correctly that I need to make a travel plan and as soon as I get on the train and my ticket is checked, I have already started using my Interrail ticket? on some trains they will ask for a seat reservation ticket and a document

You need to add your trains to your pass before boarding a train. Doing that for the first train on a day will activate a travel day and a QR code. That QR code is your ticket.

Also see:
https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

Yesterday's app update included this train, so you can now add it from the planner instead of manually.