The version of your local project doesn’t matter, what matters is the version of the dotnet tool you have installed. If you have the latest v6.0 installed you need to use studio6:
$ app open studio6 --debug
I’ve updated an issue with studio6 that will load its saved site.settings properly.
If you can’t run studio6 with v6 app tool it might be related to your possible corrupted installation, can you try removing (or renaming) your .servicestack folder and try running app open studio6 again.