There are "communication" issues currently between many brokerage firms' online statements (electronically produced statements not just "found online") and many of the tax preparation software companies this year. TurboTax is feverishly working to a resolve and the expected approximate period for this resolve as of right now is mid March. In the meantime, for expedition of e-filing, there are many suggested routes to take being "tossed around" in the TurboTax Community. Here is my suggestion:
If your statement (# of pages) is not too voluminous, delete out the imported statement and manually enter each transaction. For all transactions categorized as L/T, make sure to select a date that forces the transaction to be L/T and same for S/T (pick any date in 2020 prior to the sale date to force it to S/T). For Adjusted Basis items - this should be on your brokerage statement. If it is not, since this is historical information, do your best to accurately judge approximate time period of purchase (or inheritance, if that be the case) and search online for the cost or adjusted basis for each transaction.
If you statement is very voluminous, instead of manually entering each transaction, enter the totals for each category and follow suit with the instructions I've laid out above.
I know this is not the ideal, but unfortunately with the year we've had and the struggles/challenges brokerage firms (and tax prep software cos, not just TurboTax, truly!) are having with their software properly "speaking" to one another, this is again the most expeditious route to take for e-filing.