"I know, I forget."

Translation: Saya tahu, saya lupa.

[FresnoVegas]

Why is "aku" unacceptable here?

[Handrisuselo]

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.

[Kompot0669]

Aku is the less formal word for I or me

[icad.am]

Aku, saya, gue, aing, abdi, koe, watashi

[mybooarminarlert]

ngapa ada watashi disitu ngab, bikin ngakak aje lu

[Appers6]

When you load up Duolingo after skipping a few days...

[Serador2]

Aku tahu, aku lupa - should be accepted

[Doug264842]

The English sentence should use a semicolon or a period; a comma is only used with a conjunction.