buy the pass already but can not start the pass

Did you buy the pass from Interrail / Eurail? 
You should have a 6 digit code of letters & numbers in the confirmation email. That's your pass number, which you need to add in the app.

I checked this official video when I set up my pass recently: https://www.youtube.com/watch?v=Vl0gcQPqoW4

1. Who said that? If it's an error message then could you please post a screenshot?
2. Where did you buy your pass?
3. Did you receive a conformation mail with a pass number?
4. If you didn't receive a mail, and bought your pass on eurail.com, then do your see your pass in your Eurail account, in de Order overview? If so, then you can also see your pass number there.