TurboTax does not support tax year 2018.
If you were to file a 2018 tax return with a tax refund, the refund will not be paid since it is beyond the 3 years from the due date of the tax return.
You will need to seek out a local tax professional to assist you in completing the 2018 tax return.
You would have to search the internet for the 2018 program which your computer may or not be able to run.
Sorry, You can not do 2018 and prior using Turbo Tax. They only sell the last 3 years. You might want to go to a local accountant or tax place if you have a complicated return. For simple returns you can fill out the forms by hand. And it is too late to get any refunds. Even if you found the program to buy somewhere you would not be able to update it or download any state programs you might need.
Or If you have a simple return You can fill out the blank forms for free.
Here is the IRS 2018 Form 1040
https://www.irs.gov/pub/irs-prior/f1040--2018.pdf
And instructions https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf
IRS 2018 Schedules 1 thru 6 (only if needed)
https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf
https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf
https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf
https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf
https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf
https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf
Don’t forget your state. And you will have to print and mail your returns. So be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.