Hoe do.i download my 2021 w2

Turbo Tax doesn't have a copy of your W2.  You need to ask your employer for another copy.  Or do you mean your tax return?

If you used the Online version you should always save both the PDF file and the tax data file each year (before online closes Oct. 15) so you have a copy when you need it or if you ever need to amend it in the future. You shouldn't rely on getting a copy from Turbo Tax. Or if you want to use the Desktop program next year so you can transfer from this year.

Save the PDF
https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-pdf/00/26234

Or are you trying to import your W2 into your tax return?  How to import your W2
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-import-or-enter-my-w-2/01/26253