Do not try to transfer a pdf.   You need the tax file.

How to transfer last year’s return? https://ttlc.intuit.com/questions/1900883

The .tax2020 file must be on the C drive and you can SEARCH for the file to transfer it. 

TurboTax should be able to read from any file directory to which you can browse,

BUT 

if that doesn't work, move the file to C:

That's sloppy software.

On Windows the tax files should be in your Documents in a TurboTax folder.  When you installed 2021 it should have made that folder to put the files.  Or when you start a new 2021 return and select Transfer you can Browse to where you saved the .tax2020 file.

Are you moving them to a new computer?

What I would do is just copy the whole Turbo Tax folder that is under your Documents. That should be where the .tax files and pdf files are stored. Copy that folder to a flash drive or best yet is to burn it to a CD or DVD and then you will have a backup of them. Then on the new computer copy the folder (or files) from the flash drive to your Documents folder.

You only need the tax return data file ending in .tax2020 to transfer into 2021.

See this article on how to move your tax return to another computer…….
https://ttlc.intuit.com/community/tax-data-file/help/how-do-i-move-my-tax-data-file-to-another-computer/00/26128

I should add that I'm working on a Macintosh Mac Mini, OS 11.6

https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-mac-software/00/25609

I'm not sure why I'm hitting a brick wall with this simple procedure, but maybe because I'm using a Macintosh Mac Mini, so C: drive just doesn't register with my Mac Mind.

Your OS does not meet the system requirements for 2021 software.   Read the link that was provided to you with the system requirements.

How do I transfer last year's return into the 2021 TurboTax for Mac software?

To transfer data from your 2021 TurboTax for Mac return:

1. Open TurboTax 2021 and choose File > New Tax Return.
2. Click Begin, select Use a tax return that's on my computer, choose your return from the list, and then click Continue.
   • If you don't see your return, click Find your tax return.
   • If your 2020 return is on another computer, we've got instructions on how to move it to your 2018 computer.
3. Click Continue to review the transferred information. You can also review, edit, or delete transferred information later on.

https://ttlc.intuit.com/community/tax-data-file/help/how-do-i-transfer-last-year-s-return-into-the-turbotax-for-mac-software/00/26062

see comment above. My turbotax 2020 return  file  is in documents (TurboTab_Online file) It will not import

To clarify, are you trying to import your tax file from TurboTax Online into TurboTax Desktop?

@aldoccc

Same issue here. I just purchased and downloaded TurboTax Premier 2021 (from Amazon), installed and updated the program, and transferred last year's return (*.tax2020 file), and yet no data transfers to the new return. On a Mac running OS 11.6.1 (Big Sur). I've been using TT for the last eight years and this is the first time I've encountered this (seemingly basic) issue. My only thought is with the typically frequent updates to TT that occur early in the tax year this will be fixed, but honestly, this should never have been released with this bug.

Over the weekend, I opened up my Turbotax program, and as often happens, the system decided to reload the whole enchilada; and I believe that this time the updated program found and transferred the 2020 tax info to the 2021 form.  See if that works for you.  (Of course, the proof of the pudding will be in the eating, and only when I have received all my supporting information from banks, etc. will I know for sure.  

But I tend to agree with you that obvious, essential items can be overlooked by the programmers who are engineering all these revisions, and what is given today can be taken away tomorrow.

Best of luck!