Unfortunately, you will have to start the return from scratch, as the option to mark the person as deceased is not available when transferring a single return (no spouse) in TurboTax Online.
Please note: TurboTax Online doesn't have form T1255, which is needed if your father owned a principal residence at the time of his death. You can download it from the CRA website, print it and mail it with the rest of his return. You can find form T1255 here: https://www.canada.ca/en/revenue-agency/services/forms-publications/forms/t1255.html
