Yes, all nouns are capitalised in German.
That information is one of the first things in the tips and notes for the first unit on this course, so I'm guessing you may not know about the tips and notes which explain German grammar.
They're not available to most users of mobile apps, which is why I would not recommend using mobile apps for learning a language on Duolingo (though they can be useful for revision).
On the Duolingo website https://www.duolingo.com/ , you can access the tips and notes for a unit by selecting the unit and then clicking on the light bulb next to the 'Start' button:
If you don't see the light bulb on a small screen (such as a mobile), you could try requesting the Desktop view of the website; if that doesn't work, you may have to use the website on a larger screen (such as a tablet or laptop computer).
Thanks so much! Yes, I use the app on my phone, and have never seen that lightbulb icon. I wonder why they left it out on the mobile app. Unfortunately, it's not possible for me to use a computer or similar device right now, so I'm stuck with the phone, although it seems that using the site on the phone instead of the app reveals the lightbulb. How do I request the desktop view for the app? Do I request it on Google play? I have requested some other changes there before, like including cyrillic keyboards for people who can't access other languages on their keyboards (mine can only harbour 5 languages and I already have 5 languages set on it for regular use, so I could use a keyboard provided in the course) and courses for some new languages. Sorry if my question is a stupid one and thanks :)
"request desktop view" is something that some browsers (e.g. Safari, Chrome, Firefox) provide when viewing a website that behaves differently when it thinks it's being accessed by a mobile device compared to a desktop.
It's not something you can do from within the mobile app; it's only applicable to opening the website on a browser on your device.
All nouns are capitalised in German.
This is mentioned into the tips and notes for the very first unit -- if you did not know this, perhaps you don't know about the tips and notes? If so, you're missing out on a lot of grammar exaplanations.
They are currently not available to most users of mobile apps, so I would recommend using the website https://www.duolingo.com/ .
The tips and notes "hide" behind a lightbulb in the little window that pops up after you have chosen a unit: