Hmmm... I'm unsure why you're getting this error. It's not something we can recreate, but are investigating.
In the Facebook Developer area, in the general "Settings" area is an option for Contact Email. You're right that Support Email isn't shown there. It's only shown if you enable the App Center "Product" for your app. Many users don't enable the App Center. If you did enable the App Center and the Support Email wasn't set, then Autotune normally wouldn't let you save your settings.. so we always had it included as a required setting.
However, it's possible that Facebook has changed something to where you can't set it at all if App Center isn't enabled.
One last question, are you using a 'standard' email address, like a .com or even a large provider one like gmail.com? If you're using from a different top-level domain, like .xyz or something, Facebook may just be rejecting that email as not valid for some reason. If you can try a more common domain, that may fix the problem as well.
Thanks,
Alex