Yes, you can wait until 2020 to file your 2018 income tax return. However, if you owe additional taxes for 2018, you will pay a failure to file penalty, a failure to pay penalty, and interest. If you are due a refund, there are no penalties.
If you've filed your return and need to add additional information, then you will have to prepare an amended return in TurboTax.
If you used TurboTax Online, simply log in to your account and select “Amend a return that was filed and accepted.”
If you used our CD/download product, sign back into your return and select “Amend a filed return.” You must file a separate Form 1040X for each tax return you are amending.
I already filed 2019 taxes on TurboTax n I'm trying to file 2018
To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
No, you should file your 2018 tax returns as soon as possible. The reply by @DoninGA provides you with the information you need to file your 2018 tax return.
My understanding is that you can. I used a 1040-SR form and freefile TurboTax for 2019 tax return. I found a printable 2018 1040 form online and filled it in by hand. Looks like I send both to the same place, so I'm mailing them together. (I always have to mail my returns -- I think it's because my income is so low). My tax returns are extremely simple. Just Social Security and Standard Deduction, and nothing else. I don't owe anything. For future viewers, as I write this we are going through a Covid-19 outbreak nationwide. There has been talk of some cash payments from the government for expenses and losses due to the virus pandemic. I have read the amount of the payments will be based on income as reported and verified in 2018 tax returns. I don't have to file, but I will file every year from now on. I hope I won't miss out on any help. I haven't lost as much as many workers, but it definitely has cost me due to shortages of supplies and the need to help family. And I worry every month if I'll make it to my next Social Security payment in normal times, so anything extra means something else has to be cut. I'm keeping saved and printed copies (signed) with the mailing date. If you are able to submit electronically, you can keep track of it that way.