TurboTax does not import transactions from the .csv file that you see in your Fidelity account. It accesses a special file that Fidelity creates specifically for importing by TurboTax. Since the final extended filing deadline for 2020 tax returns has passed, I would assume that Fidelity has taken down the TurboTax import file for 2020.
You can only import brokerage data for the current filing year. Importing is never available for past years. Right now (in November 2021) we are between tax years. Filing for 2020 has ended and filing for 2021 will not start until sometime in January 2022. You can still file late, but only by mail. You can no longer e-file a 2020 tax return.
