In general:

Regular scheduled buses are not included.

Bus links provided by rail companies as replacement for suspended rail services are included.

There will be exceptions to this, If you want more specific then provide more details in your question.

IF-always due to works along the iron rails-there is bus substitution (sostituvo- SEVbus in DE, replacement in /en/) then there is usually no need-or even chance to RES. Just follow crowd-you wont be the only 1 to have to change for part of the way then. THEse buses will be shown in the usual railplanners-others that are not need to be paid for

And why can't I book a reservation for seats on trains? I'm already on the interrail and I'm having that problem. Im having to go buy bus, which it's not included in the pass, from one city to another because there are no seats left. And I tried to book them several times before and it never let me book. Is it possible to book for my next destinations or am I only able to go with bus? Thanks for the attention 

You need to let us know where you want to travel. Otherwise we can't give you any advice. 

On most routes you can travel without reservations. It will only take longer time to travel.

Where reservations are mandatory there are many ways of trying to acquire those. There are many guides already on the community of those. Reservations are generally only for express trains, night trains and Intercity trains.

There are always slower regional trains between cities that do not require reservations, but these may need a slightly different approach to searching such as setting the filter on the interrail search to reservation free journeys only. Alternatively you can break long journeys into shorter legs e.g. Venice to Milan, Milan to Ventimiglia and Ventimiglia to Nice shows many more trains than searching Venice to Nice as one journey. There are many regular express trains from Venice to Milan so opportunities to reserve are dramatically increased.

This avoids the use of buses.

If you are already travelling I assume you have checked in the station ticket office for reservations and alternative trains, both requiring reservations and regional routings. These can be added at any time before you board a train.

Yes I have. And they told me to get regional trains, but that means I would have to change train like 4 times. But my question is, why couldn't I book the seat reservation before, because now I'm forced to find an alternative that wasn't supposed to. 

@Guilherme Silva If you tell us where and when you are travelling you will get advice.