Sorry---brokerages do not make past year information available for import to tax software. You should have imported it for the tax year before it was too late. Now the only thing you can do is enter the information manually. Only 2021 information can be imported. Get the statements from your brokerage and enter the data by hand.
And....a 2020 return cannot be prepared using the online software. Online is only for 2021 returns. You need the desktop software on a full PC or Mac in order to prepare a 2020 return, which must then be filed by mail.
call them. that info may no longer be available for import. in addition, you need the desktop software for that year.
TurboTax Online and the mobile app can no longer be used to prepare or file originial returns prior to 2021 only amend them.