I'm trying to amend my 2021 tax return to include a schedule D and can't find where to access it. Where do I find adding a schedule D for an amended return in TT?

To enter an investment sale -

Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Under Investment Income
On Stocks, Cryptocurrency, Mutual Funds, Bonds, Other (1099-B), click the start or update button

Or enter investment sales in the Search box located in the upper right of the program screen. Click on Jump to investment sales

OK....to do that, for 2021 taxes, you have to be using the Desktop H&B version. 

After you start your amended  tax return, you'd navigate as follows.

1) Go to the Personal taxes section   (if you are using any of the lower software versions, it's the Federal Taxes tab)

2) Get into the Personal Income tab

3)  Scroll down to the Investment Income section

4)    Start in at the menu selection for "Stocks, Mutual Funds, Bonds, Other"

5)   Create your various Investment sales in that section, and that creates the Schedule D.