Diplomacy solo practice site
Yes, the online Diplomacy playing site Backstabbr has a "sandbox" feature that does that.

It is perfect to go through attack scenarios and see how they end up.

For practicing submitting orders, you enter orders by clicking on the map, and see how they would be written by hand. You don't submit textual orders. I guess you could write them down by hand first, and see whether the generated order is the same.