How do you pay to download a previous tax return? I'm trying to download my 2019 form and it says I did not pay for the plan that allows me to do that.

I don't think you need to pay.  Unless you used a paid version and didn't pay back in 2020 or you upgraded to a higher version.  Did you file 2020?

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 2021 for the side menu to open up. Just continue a little ways into 2021.

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

If you didn't pay or need to finish, you may need to buy the Desktop program to finish.  How to finish a return you started online

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online/01/26322