Visual Basic - HTTPClient and API Request Issue - json

Question

Good morning,
As a returning developer (I've been in management for a long time!), I've been tasked with developing a module for our proprietary financial system (Visual Basic/ SQL Server) that will read large .CSV files containing customer information, convert them into JSON format, then submit them to an external party for processing via that company's API.
The conversion part was easy and I'm almost ready to go with it, but I can't establish connectivity to the external API.
There are two parts to the process: -
Submit login details (obfuscated here) {"username": "MadeUpUser","password": "Y66***uYj6%%YY"} and receive a Bearer Token from the API
Submit JSON format customer info to API endpoint, using Bearer Token, receive confirmation
I've submitted both the login creds and my JSON-format data to the API via Postman and it works perfectly, however, when I try to login via my VB app using the HTTPClient class, I'm getting the follow exception (copied from the Exception class properties) :
**HResult = -2146233088
StackTrace = " at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)" & vbCrLf & " at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)"
InnerException = {"The request was aborted: Could not create SSL/TLS secure channel."}**
It's clearly something I'm doing wrong programmatically or something I'm not doing and any help or advice offered will be greatly appreciated.
The code: (AddLogEntry() is a Sub I created to log events to a text file)
Private Async Function PostAsync(ByVal JSONString As String, EndPoint As String) As Task
Dim APIuri As New Uri(EndPoint)
Try
With ZincClient
.BaseAddress = APIuri
.DefaultRequestHeaders.Accept.Clear()
.DefaultRequestHeaders.Accept.Add(New Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"))
End With
Dim content As New Net.Http.StringContent(JSONString, System.Text.Encoding.UTF8, "application/json")
Dim response As Net.Http.HttpResponseMessage = Await ZincClient.PostAsync(APIuri, content)
Dim result As String = Await response.Content.ReadAsStringAsync()
Catch ex As Exception
AddLogEntry("ERROR! - " & ex.Message)
End Try
Return
End Function