The TurboTax online editions can only be used for one tax return per account and User ID. You can have up to 5 User ID's associated with one email address.
You will need to create a new account with a new User ID for your father's tax return. Go to the TurboTax website for the online editions, choose an edition and create the new account - https://turbotax.intuit.com/personal-taxes/online/
If you need to create, complete and file an Estate tax return, Form 1041, then you would need to use the TurboTax Business Edition. The Business Edition can only be installed on a Windows based personal computer, not on a Mac or online. The Business Edition cannot be used to complete a personal tax return, Form 1040.
Go to this TurboTax website for the Business Edition - https://turbotax.intuit.com/small-business-taxes/
Or you can use the IRS Form 1041, U.S. Income Tax Return for Estates and Trusts to complete manually, then print and mail the Form 1041 to the IRS.
IRS Form 1041 - https://www.irs.gov/pub/irs-pdf/f1041.pdf
IRS Form 1041 instructions - https://www.irs.gov/pub/irs-pdf/i1041.pdf