Hello,
I think you use VintaSoftTwain.NET Library on the server-side - this is impossible.
Here are possible scenarios:
1. Scanner is located on client's computer, client should scan documents using web application and upload them to web server. In this case you should use VintaSoftTwain.NET Library on client-side in JavaScript or VBScript because scanner is located on client-side. Please see our ASP.NET examples here:
https://www.vintasoft.com/vstwain-dotnet-examples.html. Also you can see ASP.NET examples included in distributive package of the product.
2. Scanner is located on the server and client should have access to it. In this case you should create a client-server application or web service. Please see example 4 here:
https://www.vintasoft.com/vstwain-dotnet-examples.html.
Best regards, Alexander