Fix: Error with loading AuthorizeNet payment plugin during debug - NopCommerce
If you are facing below problem
Method 'ValidatePaymentForm' in type 'Nop.Plugin.Payments.AuthorizeNet.Controllers.PaymentAuthorizeNetController' from assembly 'Nop.Plugin.Payments.AuthorizeNet, Version=126.96.36.199, Culture=neutral, PublicKeyToken=null' does not have an implementation.
most common reason is you might have MVC 4 installed on your machine and the quick and simple fix is to rename the system.web.mvc dll of MVC 4 so that it wont conflict with it.
find C:\Program Files (x86)\Microsoft ASP.NET\ASP.NET MVC 4\Assemblies rename System.Web.Mvc.dll to _System.Web.Mvc.dll