Can I file a 2021 tax return if my 2020 tax return has not yet been processed by the IRS? How? Is there an IRS guidance?

Yes, you can file your 2021 tax return even if your 2020 has not been processed by the IRS. The two years are independent of one another.

Thank you but How? do I have to make 2020 AGI  $0 on my 2021 taxes ? Or do nothing and just file?

Is there an IRS Guidance?

For IRS guidance, you can contact the IRS.  Turbo Tax offers expert advice. 

What is TurboTax Live?
https://ttlc.intuit.com/community/choosing-a-product/help/what-is-turbotax-live/00/27191

So this is the advice of the expert to my question on guidance.  Go ask someone else. Wow! I purchase the TTdeluxe so that I could get advice on how to's and help entering in my 2021 taxes since I have not received my 2020 back and thought someone smart ,not smart ass could tell me if there was any special treatment I had to do to file.  You put the U in Obnoxious Good job TT Expert!

Yes, you can file the 2021 return provided you don't have carryover losses on the 2020 tax return.  

You can E-File your 2021 tax return.  

1. If your 2020 tax return has not completely been processed the AGI would be entered as "0". 
2. If the 2020 return has been processed the AGI can be found on line 11 of the 2020 1040 tax form.

" carryover losses on the 2020 tax return.  "

This is no reason to delay sending in your tax return now.

Do not file on paper unless absolutely necessary.

There are currently 4.9 million paper returns waiting to be reviewed and processed ahead of you.

If you expect a refund, there is no penalty for filing after April 15.