I don't wish to excuse the audio, which I have no way of listening to now, and clearly they should have audio where you can make out what they are saying, but you will discover when we do plurals in detail that it is an unusual feature of Gaelic that the best way to be certain what the vowel is is to listen to the adjacent consonant. In this case, a Gael may not hear the a' but they will know it there because there is no other way the ch could be there.
I think your instinct is right to be surprised:
I have always known this word is feminine, but I have never questioned it. Now you mention it, it is odd.
- Many m/f languages, such as French and Welsh are broadly 50/50, but Gaelic and Irish have significantly more masculine words. This means the best guess, in the absence of all clues, is masculine.
- You will find lists of semantic groups, such as diseases and musical instruments, that are usually feminine, but craobh is not in any of these lists.
- Certain suffixes, such as -ag indicate a feminine. There is no such suffix here.
- Certain terminations, such as -achd are typically feminine, but craobh is not on any of these lists' Indeed I searched for obh in AFB setting the option to 'end of word' and all the other nouns I found were masculine. As it happens, none was common enough to worry about on this course apart from taobh 'side'.
- It is more common for feminine words to have a slender final vowel than for masculine words.
- If you know the same word in another language, that can be a clue, but this word is only known (to Wiktionary) in Gaelic, Irish and Old Irish, so that doesn't help.
This means that this word evades all the clues that a word might be feminine. Even if you do not know these rules, you will be picking up an instinct for what 'feels feminine' and this word clearly feels very masculine.
Let's go back to the Wiktionary entry to see if we can find an explanation. The word comes from Old Irish. The Old Irish word is feminine. (Note that masculine and neuter nouns in Old Irish became masculine. That is the reason we have more masculine words than feminine.) But it is also spelt cráeb or cróeb (allegedly – not found in eDil). Now we see that the old spelling would make it feminine by rule 5. There is no clue as to why the vowel broadened or where the word came from, so that's where the trail goes cold.
Duolingo gets confused by apostrophes and the writers/mods on this course have found it works better not to have apostrophes at the end of words on the tiles. So I am afraid you just have to do without the apostrophe on the tile questions. I would love to know what they do on the English course with a word like cats'.