The latest update fixed SK on macOS, but it seems that the Steam version of the game hasn't been updated and still has the error discussed in this other thread: https://forums.spiralknights.com/en/node/124890
I tried replacing the app in Steam's folder with the website version which resulted in no Steam account login.
Then I tried moving Java folder from the website version into the Steam version, but that resulted in different errors.
Thanks for letting us know it wasn't working for you. We've made some changes and the MacOS Steam install should now be working. Please give it another try whenever you have the chance.