How can I gain access to my fathers account? I have his POA

Contact TurboTax support for this issue.

See this TurboTax support FAQ for contacting support - https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632

Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax

Is your dad still living?   POA is not relevant if he has passed away.   You need your father's account and user ID to access his account.   Have a discussion with customer support.

To call TurboTax customer support

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

Customer support is available from 5 a.m. to 5 p.m. Pacific time daily Monday- Friday

Here is a copy of a reply to a user from a few years ago who needed access to an account of a family member who had passed away:

Filing for relative who died

Hello @nardit1, I'm sorry to hear about your loss. I understand that this may seem like a lot to get into an account but we must take all due precaution to ensure the protection, privacy, and security of customer account information. We wish to make sure that the data your sister-in-law entrusted us with is protected. 
You will first need to fill out an Email Change Request: https://ttlc.intuit.com/questions/1899319 
In addition to your own photo ID, you must include one of the following documents: 

• A copy of the Death Certificate – The name of the surviving spouse will need to be matched to a Joint E-filed Return in our system
• A court order issuing Power of Attorney for the deceased and IRS form 56. (required for non spouse)
• A court order marking them the executor of the estate and IRS form 56. (required for non spouse)

These additional documents may be included with the initial scan of the Photo ID.
You will also need to print and mail the return and include IRS Form 56. 
I hope this helps! 

Answered by TurboTaxGabi