How can I file my deceased Dad's last return? Turbotax keeps loading my information.

Are you using the Online version?    Online is only good for 1 return per account.  But you can use the same email address for 5 accounts.   Start another Online return here and sign up for a new account

https://turbotax.intuit.com/personal-taxes/online/

The Desktop program can do unlimited returns and efile 5.  You can buy the Desktop CD/Download program at

TurboTax® 2021-2022 CD/Download Tax Software, Compare Desktop Tax Preparation

Pretty sure if you're using online you can't use your account for your Dad's return. You have to use his if he had one or start a new one for him to do his final return.