Can turbo tax complete form 8962 on a previously filed 1040-sr?

Yes but you will need to wait until the amendment program is available sometime in December 

OR  download & install the regular program 

OR  fill in the form manually :

https://www.irs.gov/forms-pubs/about-form-8962

The 2019 amendment instructions are not functional yet ... in early December when the online system opens the amendment option will be presented. 

If you used the TurboTax online editions for the 2019 tax return you will need to download the tax data file from the online account to your computer.  You will need to install the 2019 TurboTax software on your computer to open the 2019 tax data file.

If you purchased one of the 2019 TurboTax online editions you can contact TurboTax support to have them push a 2019 desktop installation file to your download account at no charge - https://support.turbotax.intuit.com/contact/

See this TurboTax support FAQ for the Form 8962 and Form 1095-A as requested by the IRS for tax year 2019 - https://ttlc.intuit.com/community/health-care/help/the-irs-is-requesting-form-8962/01/27409

A Form 8962 will be created and completed by the TurboTax software if the user has entered a Form 1095-A.  If the Form 1095-A has not been entered then the Form 8962 will not be created.

If you used the TurboTax online editions for the 2019 tax return you will need to download the tax data file from the online account to your computer.  You will need to install the 2019 TurboTax software on your computer to open the 2019 tax data file.

Then follow the procedure in this TurboTax support FAQ for the Form 8962 and Form 1095-A as requested by the IRS for tax year 2019 (Use the procedure TurboTax CD/Download Instructions for Windows) - https://ttlc.intuit.com/community/health-care/help/the-irs-is-requesting-form-8962/01/27409

If you purchased one of the 2019 TurboTax online editions you can contact TurboTax support to have them push a 2019 desktop installation file to your download account at no charge - https://support.turbotax.intuit.com/contact/

To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download .tax file