Microsoft Partner Center Package Flight 'Submit to the Store' button is disabled

I've created a new Package Flight in Partner Center for an existing app. I've uploaded a new build and it has been validated. I've set the Flight Options. Everything on the page appears good (Green checks) but the 'Submit to the Store' button is disabled.

I've tried deleting the flight and creating a new one but the result is always the same. I've submitted a ticket with Microsoft but I haven't heard back from them yet. Any ideas?

It turns out that my developer agreement had expired and I had to accept the new one. Too bad there wasn't an error indicating so on the page above! When I nagivated to the Apps and Games Overview I saw this:

Once I accepted the agreement the 'Submit to the Store' button was enabled.

Thanks Taco for pointing me to the answer!