Hi usamasattar
My name is Andre Da Costa; an Independent Consultant, Windows Insider MVP and Windows & Devices for IT MVP. I'm here to help you with your problem.
Windows Sandbox is quite new, so there might be some functionality issues to work out.
I noticed I don't have the option to enable optional features in Windows Sandbox, which is how I would enable the .NET Framework. So, this might be a current limitation of it.
I would also recommend you file a bug report; send me the short link so I can vote on it and bring it to the attention of the Windows engineers.
https://windows10.help/blogs/entry/54-how-to-su...
Information in the above link is sourced from a trusted Microsoft MVP blog.