Debug my Remote Event Receiver using Ngrok will return this error "The remote server returned an error: (401) Unauthorized

Question

I have the following code inside my remote event receiver:-

  public void ProcessOneWayEvent(SPRemoteEventProperties properties)
         {
             int listItemID = properties.ItemEventProperties.ListItemId;
             var listname = properties.ItemEventProperties.ListTitle;
             string siteUrl = properties.ItemEventProperties.WebUrl;
             using (ClientContext context = Helpers.GetAppOnlyContext(siteUrl))
             {

and the Helpers.GetAppOnlyContext is :-

         public static ClientContext GetAppOnlyContext(string siteUrl)
         {
             try
             {
                 Uri siteUri = new Uri(siteUrl);
                 string realm = TokenHelper.GetRealmFromTargetUrl(siteUri);
                 string accessToken = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal, siteUri.Authority, realm).AccessToken;
    
                 return TokenHelper.GetClientContextWithAccessToken(siteUri.ToString(), accessToken);
             }
    
             catch (Exception ex)
             {
                 Trace.TraceInformation("GetAppOnlyContext failed. {0}", ex.Message);
             }
             return null;
         }
    
         public static ClientContext GetAuthenticatedContext(string siteUrl)
         {
             string userName = WebConfigurationManager.AppSettings.Get("AuthenticatedUserName");
             string password = WebConfigurationManager.AppSettings.Get("AuthenticatedUserPassword");
             return GetAuthenticatedContext(siteUrl, userName, password);
         }
    
         public static ClientContext GetAuthenticatedContext(string siteUrl, string userName, SecureString password)
         {
             ClientContext ctx = new ClientContext(siteUrl);
             ctx.Credentials = new SharePointOnlineCredentials(userName, password);
             return ctx;
         }
    
         public static ClientContext GetAuthenticatedContext(string siteUrl, string userName, string password)
         {
             SecureString securePassword = GetPassword(password);
             return GetAuthenticatedContext(siteUrl, userName, securePassword);
         }
    
         private static SecureString GetPassword(string passwd)
         {
             var secure = new SecureString();
             foreach (char c in passwd)
             {
                 secure.AppendChar(c);
             }
             return secure;
         }
    
         public static string EmptyIfNull(object obj)
         {
             return obj == null ? "" : obj.ToString();
         }
     }

now when i add a new item >> the remote event receiver will get fired, but i will get this error :-

 The remote server returned an error: (401) Unauthorized. 

so can anyone advice on this ? Although i have generated the ClientID & ClientSecret using the sharepoint app registration page @ _layouts/15/AppRegNew.aspx and i added the following permision inside the AppInv.aspx page:-

 <AppPermissionRequests AllowAppOnlyPolicy="true">
   <AppPermissionRequest Scope="http://sharepoint/content/tenant" Right="FullControl" />
 </AppPermissionRequests>

Answer 1

For new tenants, apps using an ACS app-only access token is disabled by default.

Granting access using SharePoint App-Only

Try using the Azure AD app-only model instead.

Granting access via Azure AD App-Only