Delta definitely broke something on their side within the past month or two as there are a lot of reports of this.

There is a link from the in flight meal thread and you can see the menu with codes here:
https://cloud.o.delta.com/fltInfo
Figure out which you want, then call up ask the agent to put in your meal code choice since you keep getting an error. Also submit a complaint to DL that their app is broken and meal selection doesn't work.