If anyone is curious what's going on here from a technical standpoint, it seems the contributors entered "naaki yáál" using "a" with a combining acute accent (U+0301) instead of using the precomposed character "á" (U+00E1).
From the system's standpoint, the "yáál" on this page actually contains six characters, not four. If for whatever reason you want to recreate this six-character string, you could open e.g. Word and type y a Alt-769 a Alt-769 l. Then if you copy and paste that, it'll match what's in the system. Of course you can also just copy and paste from the top of this page :) And others have offered other workarounds (I also tried "naaki ya'a'l," which worked for me).
If I'm correct about the above, the ultimate fix will involve a contributor adding "naaki yáál" with the precomposed characters to the database. I requested that be done here: https://forum.duolingo.com/comment/34092656.
Sarah287826, Even though the usual report button will not accept helpful feedback like this, it can be reported to https://support.duolingo.com/hc/en-us/requests/new Thank you for commenting on this issue.
Tip: as per my experience in the Navajo duolingo course, when the lesson won't mark your answer correct despite using the correctly accented á, it will count it as correct if you add an apostrophe after it. In this case, naaki yáál becomes naaki yaa'l and will be accepted as correct. Super frustrating I know, but I hope this helps!
ChaseHenri: Some of the symbols used in Diné bizaad (Á,á,É,é,Í,í,ó) are the same as in Spanish. So for those, one may search online for "How to type Spanish accents" and use such a table to type those characters. As for the others, I have no work-around yet. There are several downloadable Navajo keyboards, but I haven't tried using them yet. Obviously it would be more convenient if a format for characters were used in the lessons that is similar to that used in Spanish lessons, but including all the special characters of Diné bizaad.
Hello this is the correct answer you have to do a'a'. There is a technical explanation above. This is also the 'right' answer for other similar problems, like the word for onion earlier in the course. If you actually read the lengthy explanation you can probably get through simar problems elsewhere.