You cannot add a Web Part that is in a sandboxed solution to a Microsoft SharePoint Foundation 2010 page. This problem occurs when the Web Part is inherited from the Microsoft.SharePoint.WebPartPages.WebPart class.
This problem occurs because a Web Part that is inherited from the Microsoft.SharePoint.WebPartPages.WebPart class is not supported in a sandboxed solution.
To work around this problem, use a Web Part that is inherited from the ASP.NET base class.
For more information about the base class for creating SharePoint Foundation 2010 Web Parts, visit the following Microsoft Developer Network (MSDN) Web sites: