I've had another reply that has been in moderation for over 24 hours now. I'm hoping if I don't include links in this one, it will go through.
Apple changed some things recently and it's currently possible for your app to be in a state where you can generate promo codes, but they can't be redeemed. That state is "Pending Developer Release." I spoke with dev support (1-800-633-2152) and they've put in a ticket to get this fixed.
Since this is my first app, I didn't realize that there's a state AFTER "Pending Developer Release" but BEFORE your app is actually released. When you click "Release This Version" it doesn't ACTUALLY release it until the date that you've set in the "Pricing and Availability" section. In fact, your app WON'T EVER GO ON SALE from the "Pending Developer Release" state, so this bug actually saved me some launch day embarassment.
Once you've flagged the post-apple-approved build as your verified release candidate by pressing "Release This Version" your app will briefly be in the state "Preparing For App Store" before it becomes "Ready For Sale" at which point your promo codes will work and your app will launch on the pre-defined date.
How all of this works for manually released apps, I have no idea. I can only assume that will be addressed with the incoming fix that the call to dev support initiated.