if you are not required to file in 2019 and your question is really about getting the stimulus... just file the 2020 tax return in the coming months.
The way this REALLY works is EVERYTHING is based on you 2020 income, filing status and number of children under the age of 17. What these numbers were in 2018 and 2019 does not matter.
The IRS will calculate your stimulus based on 2020 numbers; that is what is considered the ACTUAL.
The two payments were just ESTIMATES
if the ACTUAL is greater than the ESTIMATE, you will receive that amount as a credit on your 2020 tax return.
if the ESTIMATE is greater than the ACTUAL, you are not required to return the money nor does it impact your refund. Line 30 on Form 1040 will be zero.
TurboTax Online is free to start, the final price depends on the forms you need and the complexity of your return. There is a completely free version of TurboTax online available through the IRS free file program, if your income is $36,000 or less. The most cost efficient way to file (if you don’t qualify for free online filing) is usually to buy the desktop version of the program as a CD or download from a discount retailer such as Costco or Amazon. You will want the TurboTax Deluxe Federal plus One Free State version. It should be around $45-$55.
if you are not required to file a 2020 tax return because your income is below the filing limit, then you do not need to file a return. However, the stimulus credit is recalculated on your 2020 tax return and if you qualify for more then you received during the year, the extra credit will be included in your tax refund. So you may want to file this year even if your income is below the filing requirement. Using the online version, you can prepare the tax return to see what the outcome is before actually paying for it.
However, there are no versions of TurboTax that are currently up-to-date to deal with both the first and second stimulus credits. I would check back after January 8 to see if the program covers the stimulus payments correctly.