I just installed UI Builder 1.0.0-preview.11 on Unity 2020.1.6f1 and it generates 278 errors. First few are
Library\PackageCache\com.unity.ui.builder@1.0.0-preview.11\Editor\Builder\Document\BuilderUXMLFileSettings.cs(11,18): error CS0122: 'VisualElementAsset' is inaccessible due to its protection level
Library\PackageCache\com.unity.ui.builder@1.0.0-preview.11\Editor\Builder\BuilderSelection.cs(50,25): error CS0122: 'StylePropertyReader' is inaccessible due to its protection level
Library\PackageCache\com.unity.ui.builder@1.0.0-preview.11\Editor\Builder\Inspector\BuilderInspector.cs(127,16): error CS0122: 'StyleRule' is inaccessible due to its protection level
I don’t think there’s anything particularly unique about my setup. This is the first package that I’ve installed for this project.