when i type simple code such as Debug.Log("hello "); it says too many characters in character liter

This may help you with intellisense problems:

Also, try update the VSCode package inside of Unity: Window → Package Manager → Search for Visual Studio Code Editor → Press the Update button

Also, this: No suggestions in Vscode

Also, here is how to report your problem productively in the Unity3D forums:

http://plbm.com/?p=220

How to understand errors in general: