When can I start filling out 2021 tax forms? I like to estimate my return ahead of time. Doesn't look available as of 11/14/21. Thank you.

The downloaded program is already available.   Online will be available in early December.

2021 TURBOTAX

CD/DOWNLOAD (This requires a full PC or Mac—-cannot be used on a mobile device)

https://turbotax.intuit.com/personal-taxes/cd-download/

You can also purchase the CD/download from various retailers such as Costco, Amazon, Walmart, Best Buy, Staples, Sam’s Club, etc. etc. etc.

This link Where can I obtain authentic TurboTax software? has a list of authorized TurboTax resellers.

When you use desktop software, there is an additional fee to e-file a state return.  You can avoid that fee if you print, sign and mail the state return.  The state e-file fee is $19.99 and increases in March.

https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...

Although the downloaded program becomes available every year sometime in  November with the online following sometime  in December  they are NOT fully operational programs ... they are only beta versions.  The final program and the state programs are never fully operational until late January at the earliest.  

http://taxpreparationsoftware.com/

ANY tax software released by any company prior to about January 15 is only released for marketing and market share.  No program is complete and ready to file until the IRS approves their final forms and then individually reviews each program's implementation of the form.  And some tax situations aren't able to be filed until February or March, again mostly due to IRS delays.

Try these tools:

http://www.irs.gov/Individuals/IRS-Withholding-Calculator  

 https://turbotax.intuit.com/tax-tools  

http://turbotax.intuit.com/tax-tools/calculators/taxcaster 

Tax Bracket Calculator - 2020-2021 Tax Brackets | TurboTax® Official (intuit.com)