You should never activate a travel day, that is connect a journey to your pass, until just before boarding the train, otherwise you loose a travel day if you have a flexi pass and your travel plans change before you board the train.
You can't delete a travel day in the past. A travel day can only be deleted until 23.59 CET the day before the travel day.
The advice is also not to activate the pass before the first travel day as you only can deactivate the pass before 00.00 on the day the validity starts.
I can be wise to test to activate the pass, with a validity date in the future, to see that it works, and then immediately deactivate the pass.
No trips are booked in the app. Reservations are made separately and the app with your pass is your ticket.
The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice.