The following errors are thrown while building a custom add-in with Dotnet v6 using the package builder UI.

Error 1 : 

FileNotFoundException at mscorlib:
Could not load file or assembly 'System.Runtime, Version=6.0.0.0, Culture=neutral, 
PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified. 
(HRESULT: 80070002)

Error 2 : 

InvalidOperationException at Spotfire.Dxp.Deployment:
Failed to analyze C:\Spotfire\SPOT_sfire_dev_14.0.2_win\SDK\Examples\Extensions\SpotfireDeveloper.CustomDataSourceExample-upgraded_to_NET_60\bin\AnyCPU\Debug\net6.0-windows\SpotfireDeveloper.CustomDataSourceExample.dll (HRESULT: 80131509)

Linux (Node-manager)

Generating packages for .NET 6 is not supported by the UI-package builder. 
The requirement is to use PackageBuilder-Console with the pkdesc-file having appropriate arguments.
(Spotfire.Dxp.PackageBuilder-Console.exe)

There are a few examples of a pkdesc-file(Package description file (tibco.com)) in the SDK that supports multiple frameworks (NETFRAMEWORK and NETCORE /net6). 
E.g. CustomToolsExample.

pkdesc-file arguments for the framework :
------
TargetFramework: Optional
    One of the following: 

•     NETFRAMEWORK -> The package is targeted for .NETFramework. This also implies that IntendedPlatform is one of the WINDOWS alternatives.
•     NETCORE -> The package is targeted for .NETCore (.NET 6 or higher). Currently this implies that IntendedPlatform is LINUX, but in the future also WINDOWS could be supported.
•     ANY -> The package is targeted for any platform.

    The default is "NETFRAMEWORK".
    Note that this can be overridden on the command line when invoking PackageBuilder-Console.
------

This article outlines the process of building a custom add-in with Dotnet v6.

Doc: 
Package Builder console parameter reference (tibco.com)
Package description file (tibco.com)