Translation:Where is the beer that I ordered?
More about English than German: In the UK, people tend to use which where people in the US normally use that: That's the house which I like (UK). In the US, which is normally reserved for a descriptive clause: The house, which had been built in 1972, was ugly. Compare with: The house that was built in 1972 was ugly. The second sentence means the same either side of the Atlantic. The first one too, but if you remove the commas, it means the same as the second one in the UK, whereas in the US it would normally be seen as poor grammar.
For the that/which question, as a British English speaker, I always feel particularly uncomfortable putting 'that' for a person, but use either 'that' or 'which' for a non-person. However, I am probably thinking mostly about formal written English. In spoken English, it seems as though anything goes these days.
Oh boy, well Duolingo should cover this under "Tips" in the "The" section. After that though look up "German Nominative vs Accusative articles" online. There's a lot to learn. But quickly: use "den" when you would otherwise use "der" (for masculine nouns) but when the noun in question is the object of a sentence, not the subject. Ie when it's the thing the verb/action is being done to. Subject verbs object = Der Mann wirft (throws) DEN Ball.
Nobody can see what you write, so speaking about "my sentence" is not very useful.
As for misspellings, in general, misspellings that result in a real word will get counted as mistakes (for example, spelling "thread" as "threat"), while misspellings that do not result in a real word will get a typo error (for example, spelling "thread" as "threaf").