10.13 does not work with turbotax 2020?

Exactly what year model is the iMac?

see > https://support.apple.com/en-us/HT201634

If it is a 2012 or newer you can upgrade it to Mojave or Catalina.

If it is a 2014 or newer you can upgrade it to Mojave, Catalina or Big Sur.

see > https://support.apple.com/en-us/HT211683

If you want to run Turbo Tax on that iMac, then you will need to upgrade it to Mojave, Catalina or Big Sur.

To upgrade the iMac to Big Sur, see > https://support.apple.com/en-us/HT201475

If you only want Mojave or Catalina, see > https://support.apple.com/en-us/HT211683