Thanks for the report. We just really investigated what is going on. It looks like there is a bug with Facebook when saving Migration settings now and that is what is throwing that error message.
The other app values are properly being saved (at least from our testing). So, if your migration settings are incorrect from our recommendations, you'll need to set those manually in your app. Other settings should be able to be set from Autotune itself, as expected.
We've already filed a bug report with Facebook about this issue and will be tracking it as they respond:
developers.facebook.com/bugs/154532864747835
I hope that explains. Again though, if you're seeing different behavior, just let me know.
Thanks,
Alex