This is a problem with the .NET 6 preview 5 – https://github.com/dotnet/core/blob/main/release-notes/6.0/known-issues.md#preview-5
The workload manifest files were renamed in .NET 6 preview 5 which results in duplicate ids and causes the workload resolver to fail. This then results in Visual Studio for Mac showing an error about the WorkloadAutoImportPropsLocator sdk not being found.
A workaround is to remove the sdk-manifests directory /usr/local/share/dotnet/sdk-manifests/6.0.100 and then install the .NET 6 preview sdk again so it adds back the manifest files it needs.

Solution:
Delete all file in the directory of /usr/local/share/dotnet/sdk-manifests/, and reinstall dotnet 6 SDK, this issue will be solved