Interrail/Eurail is not always reliable and never a good place to book (extra fees). Not available can mean lots of things : train not available on interrail.eu, train sold out, etc.

Is it for the night train ? Then please use nightjet.com : add Interrail/Eurail as a discount. What is your travel date ? Night trains are very popular and can sell out weeks in advance. There are a few alternatives but they need to be booked ASAP.

Is it for a daytime train with connection in Innsbruck ? Then you can easily book the optional seat reservation for 3€ + the 10€ Brennerpass surcharge. There is also the possibility to take regional trains for free.