TurboTax no longer supports 2018 nor do they provide updates or state downloads. It is years too late to get a refund for a 2018 return; that refund was forfeited a long time ago. If you owe tax due for 2018, it is owed with late penalties and interest.
There might be other software companies who still offer the 2018 software. Or you could prepare the return by hand to mail in.
https://www.irs.gov/pub/irs-prior/f1040--2018.pdf
https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf