System.Web.Script.Serialization cannot be solved using Visual Studios 2017 - json

Question

I have tried looking at the similar Stack Overflow questions that are similar to mine, but none of those solutions seem to be helping my problem. So I am asking again.
I changed the Project properties to .NET Framework 4, after doing some research online people had been saying that was a better Framework. Originally, I had been using .NET Framework 4.6.1.
I tried the MSDN webpage and incorporating the namespace and assembly, but I would just get more errors.
My error is:
Error BC30002 Type 'JavaScriptSerializer' is not
defined. VB_to_JSON_One C:\Users\cmannar\source\repos\VB_to_JSON_One\VB_to_JSON_One\Module1.vb 8 Active
And my warning is:
Warning BC40056 Namespace or type specified in the Imports
'System.Web.Script.Serialization' doesn't contain any public member or
cannot be found. Make sure the namespace or the type is defined and
contains at least one public member. Make sure the imported element
name doesn't use any
aliases. VB_to_JSON_One C:\Users\cmannar\source\repos\VB_to_JSON_One\VB_to_JSON_One\Module1.vb 2 Active
I am using Visual Studios 2017 and am trying to convert .NET to JSON as shown in the following code:
Imports System.Web.Script.Serialization ' Sytem.web.extensions.dll
Imports System.IO
Imports System.Text
Module Module1
Public serializer As New JavaScriptSerializer()
Sub Main()
Dim fileData() As String = File.ReadAllText("JSON_Data.txt").Split(vbLf)
Dim datapoints As New List(Of DataPoint)
For Each jsonData As String In fileData
If jsonData.Trim() <> "" Then datapoints.AddRange(getDeserialisedChargeHRData(jsonData))
Next
Dim sortedDataPoints As List(Of DataPoint) = datapoints.OrderBy(Function(o) o.dateTime).ToList()
Dim sb As New StringBuilder()
sb.AppendLine("Date and time,BPM,Confidence,Calories burned,Default zone,Custom zone")
For Each dp As DataPoint In datapoints
sb.AppendLine(String.Format("{0},{1},{2},{3},""{4}"",""{5}""", dp.dateTime, dp.bpm, dp.confidence, dp.caloriesBurned, dp.defaultZone, dp.customZone))
Next
File.WriteAllText("BMP_Data.csv", sb.ToString())
End Sub
Public Function getDeserialisedChargeHRData(ByVal json As String) As List(Of DataPoint)
Dim deserializedResult = serializer.Deserialize(Of List(Of RootObject))(json)
Return deserializedResult.Item(0).dataSets.activity.dataPoints
End Function
End Module
Public Class DataPoint
Public Property bpm() As Integer
Public Property confidence() As Integer
Public Property caloriesBurned() As Double
Public Property defaultZone() As String
Public Property customZone() As String
Public Property dateTime() As Date
End Class
Public Class Activity
Public Property dataPoints() As List(Of DataPoint)
Public Property granularity() As String
End Class
Public Class DataSets
Public Property activity() As Activity
End Class
Public Class RootObject
Public Property dataSets() As DataSets
End Class
If anyone has any suggestions or ideas, it would be greatly appreciated!