I’ve had this same issue before when repeatedly searching for flights in the DL app over a specific range of dates between two cities over a period of days/weeks/months. Any modification to the city or dates will allow me to move forward, but the original throws up the same error message. I think it’s somehow that the repeated searching essentially gets to a point it is blocked. I know that’s not a technical answer, but it’s the best I can think of.

My work around was to use the mobile delta.com website via my phone’s web browser and that allowed things to be booked.