I've gone through the German lessons one by one, however, my new lesson "Adverbs1" is inactive. Why?
Please provide a screenshot so we can see that what you are saying is the case. You can also try refreshing the page just to be sure it's not a temporary glitch. Odds are you haven't completed all the sub-lessons within the lessons preceding Adverbs1.
If it's a real problem, a lot more people would probably be encountering it by now. However, the Duo team will probably take a look if you include more details/proof of the problem..
If you're unable to share a screenshot, please post in this Troubleshooting forum and someone from Duo can take a look for you, perhaps - https://www.duolingo.com/topic/647
Most of us here are users like you, so we can't see your screen. There isn't a lot more we can do to help, unfortunately.