I have an Arduino Mega 2560. It has 3 serial ports. The standard is "Serial", the others are "Serial1" and "Serial2". VSCode doesn't have a problem with "Serial", but shows an error for "Serial1". Code compiles and uploads just fine, but would be nice not get this error message.

Version: 1.36.1 (user setup)
Commit: 2213894ea0415ee8c85c5eea0d0ff81ecc191529
Date: 2019-07-08T22:59:35.033Z
Electron: 4.2.5
Chrome: 69.0.3497.128
Node.js: 10.11.0
V8: 6.9.427.31-electron.0
OS: Windows_NT x64 10.0.17134

Name: Arduino
Id: vsciot-vscode.vscode-arduino
Description: Arduino for Visual Studio Code
Version: 0.2.27
Publisher: Microsoft
VS Marketplace Link: https://marketplace.visualstudio.com/items?itemName=vsciot-vscode.vscode-arduino