Turning off "Language Service Disabled" error message in VS2017

This helped me. You can have a try.

 Go to Tools -> Options -> Text Editor -> JavaScript/TypeScript -> Language Service -> General

and uncheck the box: "Enable the new JavaScript language service.

To solve this issue do the following:

• Create file in root directory of your project and call it tsconfig.json
• Add this:

{
  "compilerOptions": {
    "allowJs": true, 
    "noEmit": true, 
    "module": "system",
    "noImplicitAny": true,
    "removeComments": true,
    "preserveConstEnums": true,
    "sourceMap": true
  },
  "include": [
    "scripts"
  ],
  "exclude": [

  ],
  "typeAcquisition": {
    "enable": true 
  }
}

Please have a look at the below two links for tsconfig.json explanation, because you may still need to change it according to your setup. This is the only way that worked for me. I hope that will help.
https://www.typescriptlang.org/docs/handbook/tsconfig-json.html
https://developercommunity.visualstudio.com/content/problem/8148/javascript-intellisense-not-working.html

I have found a solution for this problem.

I reset my userData using:

devenv.exe /resetuserdata

Since doing this the JavaScript settings seem to have persisted and I no longer get the language service error above.

TAKE NOTE: This will reset all your user data and customisations.