If "bisa" means also "able", doesn't "I don't know how to" work? Like "I don't know how to ride a bike" = "I can't ride a bike" = "I am not able to ride the bike"?
"I don't know how to" translates to "Aku/Saya tidak tahu bagaimana/cara(nya)". It is a synonym of "I can not" but strictly speaking, it has a different shade of meaning and not interchangeable with "I can not".
"Could" and "can" are strictly speaking not interchangeable right? I'm not a native English speaker so maybe I'm wrong...
Yes, they have slightly different meanings, particularly in the tense. But, as Indonesian doesn't distinguish tense (only aspect), they can both translate to "Saya tidak tahu". However, Duo seems to be speaking in the present tense for this lesson.