Why is "aku" unacceptable here?
It should be acceptable. Maybe the participants (who contributed to translate Indonesian lessons) rarely used it, so the word "aku" was not saved in the Duolingo database.
I mean there is no the alternate translation "aku" in the database.
Aku is the less formal word for I or me
Aku, saya, gue, aing, abdi, koe, watashi
ngapa ada watashi disitu ngab, bikin ngakak aje lu
When you load up Duolingo after skipping a few days...
Aku tahu, aku lupa - should be accepted
The English sentence should use a semicolon or a period; a comma is only used with a conjunction.