Of course, I’m happy to help!
It doesn’t look like you are passing an audience param in your Flutter code - If that’s the case then you’ll receive an opaque access token. Some more on the audience param and opaque tokens can be found here.
The audience used in your Flutter app should be the same as the audience specified on your backend - A note on that: It looks like you have the management API set as your audience in your backend. I assume you are protecting your own API? In that case the audience should be whatever name (API identifier) you gave your API when registering it in Auth0.
Keep us posted!