I would like to download my 2021 tax return , but I cannot find it in my account

How to Access prior year online returns
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/27010

If you can't get the side menus to open up to access the prior year..... You need to start entering some basic Personal Info in 2023 for the side menu to open up. Just continue a little ways into 2023. I had to go though about 12 screens.

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online. So you need to make and keep your own backups.

Or request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript

Or get a copy of your return using form 4506
https://www.irs.gov/pub/irs-pdf/f4506.pdf

You might have another account.  How to view all your accounts
https://ttlc.intuit.com/turbotax-support/en-us/help-article/account-management/many-intuit-accounts-turbotax/L9aVfKS1Z_US_en_US?uid=ll5g6zcx

It's common to end up with multiple accounts. First LOG OUT of whatever TurboTax account you're logged into right now. Then use this TurboTax account recovery website to get a list of user ID's for an email address. Run the tool against any email addresses you may have used
https://myturbotax.intuit.com/account-recovery/