I am a CPA. Can I sign a turbotax return as preparer?

No.   TurboTax is for users to prepare self-prepared returns.   It is not for use by paid tax preparers.   If you want to use TT then look into the professional software.

https://proconnect.intuit.com/?s_cid=TT.com_MoreProds-Footer_ProTaxSoftware

No.  TurboTax is not licensed for use by paid preparers.

Desktop End User License Agreement

https://turbotax.intuit.com/corp/license/desktop.jsp

Scroll way down under

B Software and service terms

      1. Supplemental License Grant and Restrictions

               d.  You are not licensed or permitted to use the Software, or to allow the Software to be used, to prepare tax returns, schedules or worksheets on a professional or commercial basis (i.e., for a preparer's or other fee or consideration).