Just got an answer back from Canvas support chat: 

From Khan Academy support:
"I'm afraid that Khan Academy's exercises and courses cannot be embedded outside of the platform, and must be completed through the Khan Academy website. Any grades or scores achieved in Khan Academy cannot be reflected onto an LMS (or even Google Classroom) either. We are making numerous changes as we transition to a new technical platform. As part of this work, we stopped supporting the public API in 2020. As a result, any calls that you make to our API after that point will return errors that you already have encountered.
Khan Academy does not currently support integrations with any LMSs, including Canvas. That said, you should be able to embed URLs of Khan Academy videos hosted on YouTube and, additionally, you can always link directly to any lesson or exercise URLs found on our site and have it redirect your students from the LMS to khanacademy.org."