If you weren't required to file a tax return and didn't file any as a result, you don't need them. Just make sure to tell us that you didn't have to file a return. You can do so in the questionnaire and again when you get to the "upload tax return" step in your my.upsolve account.
If you're not sure whether you should have filed a return for the last two years, the IRS has a guided quiz online that helps you find the correct answer. You can access this "Interactive Tax Assistant" here.
If it turns out that you needed to file your tax return(s) for the last two years, you'll need to do so before you can move on to the next step. Once you've filed your tax returns, come back and upload the digital copies. More on that here.