There’s a pre-order thread here that’s slipped off the first few pages (n.b. I searched “order,” “pre-order,” and “preorder,” none of which worked):

Pre-ordering Meals on Delta, the Definitive Thread

I lost count of the number of items of agent mis-information you were given but the Wiki in the linked thread summarizes the rules well. Short version: absent a sad box market, you should be able to pre-select 7 days in advance for domestic F. Your specific aircraft type is, unfortunately, a known glitch and doesn’t allow advance meal selection.