"I wrote it in my notes."
Translation:Saya menulisnya di catatan saya.
Actually, it is correct. I don't know what might be their exact reason to mark it incorrect, but I guess it's the "ku" part at the end doesn't fit really well. "-ku" is like coming from the shortened version of "aku" to express that something belongs to the "I". But "I" generally can be translated in two versions: "saya" and "aku". Saya is a more formal form, you use it to talk to maybe a stranger, someone older or honored like teachers and professors. Meanwhile aku is a less formal form, you mostly use it when talking with friends or someone of around the same age as you, or younger. And in this case, "saya menulisnya di catatanku" might not be exactly correct because you mix saya with aku in the same sentence. If you want to use "ku" at the end, "aku menulisnya di catatanku" should be a better translation.