It would probably help to see the code. The error suggests that the ASP page is calling a .NET assembly. Is it a standard ASP page?

It not standard, it is a .net. The code for the NewScreen page is : 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Sage;
using Sage.CRM;
using Sage.CRM.Blocks;
using Sage.CRM.Controls;
using Sage.CRM.Data;
using Sage.CRM.Errors;
using Sage.CRM.Graphics;
using Sage.CRM.HTML;
using Sage.CRM.UI;
using Sage.CRM.Utils;
using Sage.CRM.WebObject;
using Sage.CRM.Wrapper;

namespace NewOpportunity.Pages
{
public class NewScreen : DataPageNew
{
public NewScreen()
: base("Opportunity", "oppo_opportunityid", "OpportunityDetailBox")
{
UseTabs = false;
}

public override void BuildContents()
{
AddError("My Host Is" + Dispatch.Host);
base.BuildContents();
}

public override void AfterSave(EntryGroup screen)

{

string elecSub = Dispatch.EitherField("oppo_elecsub");

if (elecSub == "electSub1")

{

UrlAfterSave = $"http://{Dispatch.Host}/{Dispatch.InstallName}/eware.dll/Do?SID={Keys.GetKey("SID")}&Act=115&Mode=1&CLk=T&Key-1=4&Key0=7&Key7={Id}&B=WholesaleBlock&OBN=WholesaleBlock&J=WholesaleBlock&T=Opportunity";

}

base.AfterSave(screen);

}

}

}

Are the versions of .NET correct on the dev and target machines?

Omg... I was building it for the wrong framework. Thanks for the help!

We've all done it! I'm glad the fix was that simple!