Foreword: I know this is in the main Duolingo section, but since there is no Japanese discussion section yet, I decided to do it here. (Edit: Has been moved to Japanese discussion section. Took some inspect element editing. Thanks to MyaRexa for telling me.)
There is a sentence that is often impossible to answer correctly in the "Hobby 1" lesson. Looking at the comments, it has apparently caused many people to get the question wrong. The issue is also not one of the reportable options, so it doesn't look like it's getting fixed.
It is a question where you need to translate the sentence "I always eat one slice of bread for breakfast." to Japanese. It is one of the questions where you select word tiles and put them in order, but as can be seen in the comments, the word の does not show up in the word tile options, making it very hard to make it a proper sentence. I didn't get the word 朝ご飯 (breakfast), which is obviously vital to making the correct sentence.
I hope one of the Japanese moderators sees and fixes this.
If this is the same sentence that you did, then all the components of the answer are there; in hiragana and divided between tiles, but they're all there (and as you can see, the answer doesn't really need の). If the sentence components you found were different then apparently it's been fixed.
Sorry, but just saying it is wrong without documentation or concrete examples requires us to take a leap of faith that you are right and Duolingo is not (or the iteration that you are using). Maybe if you linked an image from imgur (like arachnje has) and show us what you are talking about it would take the pressure off of the people trying to help you and make the answer less conjecture.
You probably are talking about this one:
No one could have known what happened as you don't have a screen capture or something. Duo has some problems here and there but frankly I truly believe the program is not as bad as that. I don't remember missing any tile in that type of questions (except in the case Duo's translation is bad and had really missed a word in their answer.) Is it because you are not familiar with words without Kanjis (as it is shown in the discussion thread)? Sometime Duo cut up the words not so logically and you need to assemble the Kanas yourself.
Take a screenshot next time.
So here's the deal with counters: They don't need any particles if you put them in their standard position right before the verb!
For example: トーストが 一枚（いちまい）あります。
くるまが 五台（ごだい） あります。
Yes, you can take all of these and re-arrange their order, but then you need the 「の。」And the counter would go in front of its noun. I'll put spaces in and keep the kanji to a minimum so that you can analyze it better.
一枚の トーストが あります。
五台の くるまが あります。
六匹の さかなが います。
三本の えんぴつが あります。
Practice counting things around your house and it will get more natural. If you don't know the Japanese word, that's okay. You know the shape, so just "code switch" -- Stick in the English word, put a 「が」after it, then the counter, then an あります(assuming it is an object or plant) or います (if it's a pet or a family member), and just keep practicing until it becomes more automatic. Hang in there!