Trying to open a .sit file on macOS Big Sur

Have you tried The UnArchiver from the Mac App Store? The .sit container is so old that you may not have an application that can read its contents once it is unstuffed.

If there is no newer app, you could use an old Mac or emulator like SheepShaver, Basilisk II or Mini vMac to open the .sit with some version of StuffIt (AFAIR .sitx was a newer format, did Compact Pro open .sit or was it so that StuffIt Expander opened also .cpt?).

I tried the following emulators a year ago in Mojave and they were surprisingly good and FAST. Some tinkering is needed to get them to work.

https://sheepshaver.cebix.net/

https://basilisk.cebix.net/

https://www.gryphel.com/c/minivmac/

check out SIT File Extension - What is a .sit file and how do I open it? (fileinfo.com)