I have the same issue. Been trying to install multiple times over the last month. Eventually went down the correct rabbit hole.
At least for me, I am using duckdns and I had not set the internet address for the Home Assistant URL (under Settings>System>Network). My issue with installing this went away after that.
I was also unable to install another Integration, and that helped me identify what the problem was…Official SmartThings Integration: Error Config flow could not be loaded: 500 Internal Server Error Server got itself in trouble
Good luck