macOS version 10.15 definitely supports the current 2020 version of turbotax for desktop. The problem would be that you are using the online program. The online program is “free to start“ and you only pay when you finish and file your return, so at this point if you are using TurboTax online, you should not have paid for anything yet. You can purchase a download of the desktop program from Amazon, Staples, Best Buy, or direct from TurboTax.
I’m not sure what you’re going to get if you file during the day. I have never heard that the customer service will give you premier functionality online for the deluxe price. Maybe that’s a customer service exception that they are willing to offer you.
turboTax desktop usually supports the current Mac version and one prior version, so I would expect that next year‘s 2021 Turbotax would support Big Sur but would still support Catalina 10.15. Although that would be subject to change until the version support is officially announced.
