If you placed an order and none of the Thanksgiving Dinner items were purchased, they should still be in Ibotta for you to buy. You can try another Walmart or go to the store and purchase them at the register.
Also, you should have received an email for your Walmart order. This can be used to verify any purchase (though it sounds like you didn't get anything).
I don't understand how Ibotta thinks you already ordered. It doesn't work that way. You buy it at the store, Ibotta matches it up and reimburses you for the amount. If it didn't match, then you didn't buy it and it should still be in the Ibotta app.