"Saya menulisnya di catatan saya."

Translation: I wrote it in my notes.

[JamesTWils]

I wrote it IN my notes.

[Rick392366]

Thank you.
It's accepted now.

[glhaynes90]

Should also be noted that in this case, "I wrote it on my notes" is not a correct translation at all. That would be a closer translation of "Saya menulisnya di atas catatan saya".

[Aleksandro_RU]

I hear everything but di

[I8mienaB]

why can't it be present tense? I am writing it in my notes

[boringtomi]

"I write that in my notes" should be accepted too?

[nkdijah]

Maybe, but in this sentence, "-nya" is used as a replacement for a third party, just like "it" in English, which I think "it" is a more common version to replace the third party/object than "that", unless you mean to refer to something more specific, then "that" is used. In this case when in English you use "that" so you say "write that", in Indonesian it translates as "menulis itu".