If you are receiving a message that your return has already transmitted, then you are not able to make any changes. Please check your e-filed status to see if your return has been submitted.
Depending on which TurboTax product that you used to file your taxes, there would be different ways to check the status. Also, are you checking your e-file status or the status of your refund?
Please click the link, listed below, for the TurboTax FAQ to learn how to check your e-file status and the status of your refund. https://ttlc.intuit.com/community/return-status/help/how-do-i-check-my-e-file-status/00/25538