Always returns INVALID

[ItzMeDry]

General information

• Environment: Sandbox
  Language, language version, and OS: ASP.NET VB .NET Framework 4.7.2
  Run Environment: Windows 10 version 1903, Google Chrome version 81.0.4044.129 x64

Issue description

Code: https://pastebin.com/9fyb9bJi
Description: Every time when I complete payment from paypal sandbox, it will redirect me to the page of payment success page on my website. But it always returns INVALID (My website alert shows error1, means it returns INVALID.) I have already enabled "Auto return" and "Payment data transfer", I don't know why it always returns INVALID.