Coinbase no longer supports a direct import of a CSV file into TurboTax from their site which has resulted in an error message for some users. Instead, Coinbase is relying on a 3rd party CoinTracker, as an option to calculate your cryptocurrency gains and losses and to access your CSV file. For more information, including a link to CoinTracker, please see this TurboTax help article provided below:
Coinbase no longer supports the export of Capital Gains. They are directing customers to use Cointracker for this feature. Cointracker provides the option to download TWO types of CSV files, Transaction History and Capital Gains. The Capital Gains CSV file is the one that will upload into TT correctly. This can be found by going to the "Taxes" tab at the top of the screen and selecting "Tax Center." Then, scroll down until you see "Tax Reports" on the right hand side of the screen. Click on "CSV Reports" and download "Capital Gains CSV." This should result in a successful upload of your transactions into TurboTax.