working on this myself. This is because you are in a sandbox. You can only post to private TikTok accounts for testing. Must get authorized on production to post to public accounts.
However, I'm working on a similar thing. I'm in sandbox so this is expected. I've:
made my account private - same error
reissues api keys/openIDs - same error
tried posting a draft - same error
Not sure if I've done something wrong elsewhere.