Thanks for contributing to our forum. I see that you have posted several listings successfully since you started this thread so I assume you were able to work around this issue, but I figured I would update this thread anyway in case I am mistaken or the information may be useful to other sellers who are encountering similar errors.
Based on the error message you mentioned receiving, it sounds like you had chosen Flat Rate shipping and selected a domestic shipping service, but did not specify a cost for that shipping service in the provided “Cost” field.
Provided that was the case, you should have been able to post the item successfully after filling in the “Cost” field next to each domestic shipping service you have chosen to offer.
I hope this helps.