I've been browsing the website englishgrammar.org, and there is a contact page where you can report errors to the owner, Jennifer Frost.
Because it seems there are a couple of discrepancies that need to be weeded out. For example, question 6 in Articles with uncountable nouns
- We are having ……………………………. weather.
a) a terrible b) terrible c) the terrible
the correct answer listed is:
- We are having terrible weather.
In question 10 at Countable and uncountable nouns exercise
- We are having …………………… (a terrible weather / terrible weather)
the correct answer has no indefinite article.
- We are having terrible weather.
I did the same online quiz as the OP and ...

and the result was the identical to that of the OP's

So, if you read the pages about articles and countable and uncountable nouns, the answers say "no article" is needed before terrible weather, but that is contradicted by the answer on the online quiz.
That's not the only mix up I found. Note that the answer I chose was no article is needed. Reading the explanation increased my confusion

The hint says that in British English no article is needed in front of hospital, but in American English the article, the, is always required. Which meant my answer was correct! No? Maybe? Yes! Yes, if I am speaking British English.
Articles with hospitals, schools, prisons etc.
For example, Americans usually say someone is in the hospital, much as they could be at the bank or in the park. To the British this sounds like there is only one hospital in town or that the American is thinking of one hospital in particular that he or she patronizes. The Brits say an ailing person is in hospital, just as they would say a child is at school or a criminal is in prison. This is because they are thinking more of the primary activities that take place within those institutions rather than the buildings in which they are housed. If, however, you are merely visiting one of these places, you are at the hospital, at the school or at the prison — both British and Americans agree here that what we have in mind is the building itself.
David Appleyard.com
And finally, if you work in Germany and meet somebody who is German, you would say "I met a German guy/girl yesterday." The indefinite article would be preferred because we know that Germany has more than one German man or woman. The online quiz says that either a or the works if you happen to fall in love with one of them. I'm not going to say that using the in this case is ungrammatical, because it's not; but without any background information, or context, selecting the is less probable.

1If you have Practical English Usage (Michael Swan), 3rd ed., check out entry 149.4, but eventually you have to get familiar with each of these nouns one at a time to gauge whether it's idiomatic or not to use it with the indefinite article. Most of these are arbitrary, e.g., I have a sleep doesn't work in AmE, but fine in BrE, while I have a good sleep is fine everywhere, and yet we'd say How much sleep did you get last night? (not a sleep) – Damkerng T. – 2016-10-18T20:33:19.417