You try the Web Platform Installer to download SEO toolkit on your Windows 10 or Windows Server 2012 R2. After the installation of Web Pi you got an error. That SEO Toolkit can't work on IIS before 7.
The SEO toolkit is compatible with IIS 7 or higher. This is the known bug of SEO Toolkit and until this post is submitted, there is no plan SEO Toolkit will be updated.
In order to fix this issue, please do these steps: