A "non-covered tax lot" is one where the cost basis was not reported to the IRS. You are responsible for reporting an accurate cost basis on your tax return, but because it is a non-covered lot this basis information is probably not on the 1099-B that your brokerage generated.
My guess is that TurboTax is filling in $0 as the cost basis because it doesn't know any better, which is causing your tax to be calculated as though the entire proceeds of your sale was a capital gain. You should look back at your records to determine the actual cost basis and then figure out where to enter this information into TurboTax, so that you only pay tax on the actual gain rather than the entire sale amount.