The online program is only for 2021 returns. You cannot get the same results using the 2021 software as your accountant got for a 2020 return even with the same information. The tax laws for 2021 were different from the tax laws for a 2020 return, so the results will not be the same. You would not get an accurate comparison. Each tax year has its own software which is written to comply with the tax laws in effect for the specific tax year.
In order to prepare a 2020 return you need the 2020 software, which is now only available as a CD/download.
You can try purchasing the 2020 CD/download from Amazon or some other retailer from whom it might be cheaper than it is on the TT website.