Ceciliabr, are you sure you use the installer SweetHome3D-6.3-macosx.dmg?

Yes. The installer i downloaded is 67.3 MB and is called SweetHome3D-6.3-macosx.dmg. Unless some switch has been made, I'm running the correct version.

And obviously I'm not the only one experiencing this. I even tried it on my daughters Macbook air with Catalina, with the same result: