FIXED ON OS CATALINA:
The fixes Aspyr are recommending don't work for OS Catalina. However, if you're running Catalina and still need a fix, I've figured out a workaround.
If you go into the system files, and find the location: /Users/<USERNAME>/Library/Application Support/Steam/steamapps/common/Sid Meier's Civilization VI/Civ6.app/Contents/MacOS/Civ6_Exe
(obviously putting your system username where indicated)
Then rename Civ6_Exe to AppBundleExe
For me, this fixed the launch problem; game now works, and all the DLC are there.