The rail planner app can't book anything. You can only register your journeys in there, which is the digital equivalent of writing a train in the trip report of a paper pass. By doing that, nothing is booked. There's also no need to do that a log time in advance. The only requirement is that you do it before you board your train.
Although the rail planner app contains a lot of timetables, it is incomplete and not updated frequently enough to be reliable enough for planning purposes. To plan, it's best to use national apps or websites or the operators’ own apps/websites. For international travel, bahn.com or the DB Navigator app is a good choice.
To book reservations, you can use the DB app for journeys from/to/in Germany. tickets.oebb.at also works and is cheaper when booking only 1 train. An overview of where to book is here:
https://www.seat61.com/interrail-and-eurail-reservations.htm