Use the rest of my international ticket for home-country travel?

Hi,

I live in Germany and booked a 7 day international ticket. I already went to France and back, so my two days of going inbound and outbound are used.

Now I wanted to use the remaining days from tomorrow on to travel inside Germany. I thought as I was not going outbound that would be okay. Apparently it is not, because the App wont let me validate my trips. Can you tell me what the problem here is? 

Because from my understanding the only limit was crossing the boarders to and from my home country.

Thanks a lot

Jonas