"The jacket of the boy."

[jeanprendiville]

Τζάκετ ?

[starry1086]

I wrote η ζακέτα του αγοριού and it didn't like it.

[aaronxiii]

Same, although I think it's correct.

[L.Alexander]

It's not in my dictionary, but ζακετα is. (My keyboard doesn't do accents.)

[D_..]

You may want to read the comments in this discussion: https://www.duolingo.com/comment/17496123 I suggest doing an image search, it's very reliable.
What keyboard layout do you have? On a standard QWERTY, accents are added by hitting first the key next to L, then the vowel (; or : + vowel): ;+α=ά.

[StSB6]

Why we didn't put "των" ?

[Apogeotou]

Των is the plural masculine article in genitive, whereas του is the singualr masculine article in genitive. So if we were to use των, we'd say "Το μπουφάν των αγοριών" (The boys' jacket).