The code in your installation must be OK, as this would generate an error log entry, but such an error would cause an API request to 404. Double check your actual code just to be sure.

I’ve no experience with POST endpoints. That said, maybe you should try adding a ‘permission_callback’. I don’t know if it’s actually required, but it would make sense if it were. Not just anyone authenticated should be allowed to add data, there are usually further restrictions that need to be checked.