Can't build: getting error "Invalid target framework 'unsupported'"

[fandrei]

I start build using runbuild.ps1 and getting the following error. Am I missing something, or it's broken?

Importing psake
Starting build.ps1
psake version 4.4.2
Copyright (c) 2010-2015 James Kovacs, Damian Hickey & Contributors

Executing Clean
Setting location to D:\Projects.ThirdParty\.NET\Newtonsoft.Json
Deleting existing working directory D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Working
Creating working directory D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Working

    Directory: D:\Projects.ThirdParty\.NET\Newtonsoft.Json

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
d----          2024-10-01    18:29                Working
Executing Build
Found 1 enabled builds

    Directory: D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Build

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
d----          2024-10-01    18:21                Temp
Downloading dotnet-install.ps1
dotnet-install: .NET Core SDK with version '8.0.300' is already installed.
dotnet-install: Adding to current process PATH: "C:\Users\Andrei\AppData\Local\Microsoft\dotnet\". Note: This change will not be visible if PowerShell was run as a child process.
dotnet-install: .NET Core SDK with version '6.0.400' is already installed.
dotnet-install: .NET Core SDK with version '3.1.402' is already installed.
dotnet-install: .NET Core SDK with version '2.1.818' is already installed.
MSBuild path C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\Bin\MSBuild.exe
Restoring packages for net6.0 in D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Src\Newtonsoft.Json.sln

MSBuild version 17.11.9+a69bbaaf5 for .NET Framework

  Determining projects to restore...
C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\NuGet\NuGet.targets(174,5): error : Invalid restore input. Invalid target framework 'unsupported'. Input files: D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Src\Newtonsoft.Json.Tests\Newtonsoft.Json.Tests.csproj. [D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Src\Newtonsoft.Json.sln]
Error: 2024-10-01 18:29:09:
 +                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [<<==>>] Exception: Exec: Error restoring D:\Projects.ThirdParty\.NET\Newtonsoft.Json\Src\Newtonsoft.Json.sln
build.ps1 failed