"He really doesn't know" isn't accepted. There's no subject specified, so you, the course makers, should allow for a range of possible answers: you don't know, they don't know, she doesn't know, we don't know, it doesn't know...
That's a problem with this course. Better just flag whenever you encounter a missing alternative. The course will get out of beta sooner this way. Thanks for your help.
"Why did you learn korean?" "What will you do with it?" "Why not german/spanish/etc?"
I can just imagine this in a conversation: "so, what inspired you to create this fabulous masterpiece?" "I really don't know."
Why not "Really, I don't know"