If you are using the source code version of nopCommerce you can run it in Debug (pres F5), do what you have done to cause the error and when the exception is thrown the VS (latest version) will automatically stop on the place where the exception have occurred, from there you will be able to see what exactly is the problem using the exception message and the stack trace.
If you are not able to figure it out yourself we will be happy to help, but we will require the stack trace and the exception message.