Only Customer Support can help you with this. You need to contact Customer Support through this form and clearly write what has happened. Customer support is currently overloaded with requests so you will probably have to be patient to get help. 

https://eurail.zendesk.com/hc/en-001/requests/new

Thanks, i have already completed the form. Just wanted to get through this as fast as possible

When you choose the start day of the validity of the pass, the first day of the validity period is automatically made a travel day, even if you don't enter a journey, the advice is therefore 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, which you now are well aware of.

The advice is also never to activate a travel day until just before boarding the train, otherwise you might loose a travel day if your travel plans change in a late stage  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.

Do you think support will be able to help me? 

I have no idea what the policy of Customer Support is in cases like this. Does anyone of you know @rvdborgt @seewulf 

Customer support can add travel days. They have done so in case of bugs that cause extra travel days to be used. They may also do so in clear cases of mistake, but no guarantee of course.

There's always a warning when you're about to activate a travel day, about whether you can undo the action later. That warning is there for a reason. It's a good idea to read it.

I like to compare activating a travel day with writing a new travel day on a paper pass. I would usually only do that after (!) boarding my first train of the day, since that is irreversible. With a mobile pass, you can cancel a travel day that hasn’t started, but I still don't activate in advance, to prevent mistakes as much as possible.

Hello, I have a question slightly similar.  I activated my pass (19th June) via Mobile app as I thought I needed to do this in order to reserve seats. However I did put down my travel date as 23rd July.  When I realised my mistake I deactivated my pass. My question: As I bought a 1 month pass does my pass starts when I activated my pass or from the day my ticket is scanned at the station on my first day of travel?

When you activate the pass you select a start date. The start date is the day the validity of the pass starts. Please note that the start date will be a travel day even if there is no journey connected to it.

The advice is not to activate the pass in advance as your travel plans might change. The pass can only be deactivated until 23.59 CET the day before the start date. 

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.

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.