Difficult to say without more information. What happens if you enter your ID number? Is there an error? Do you have a screenshot?

But if activation doesn't work, it's best to contact customer support. @Nanja?

Hi Dario, you will need to enter the mPass number. We see you have been able to add and activate your Pass in the meantime. If you are still experiencing issues, please contact our Customer Support team directly. 

I can’t activate our mobile passes with a first travel date end of August. The app won’t activate passes after July. I would like to start making reservations now but can’t until the passes are activated. I contacted customer service two weeks ago but still haven’t received a reply. Do you think I can activate the passes in July for an an August first travel date? Thanks.

You don't need an active pass to make reservations. 

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. 

The advice from the experienced travellers is 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.

It 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.

The advice is also to 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.