insufficientPermissions

I am receiving this error message while trying to insert a new post at 
Blogger through App script:

{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "insufficientPermissions",
    "message": "Insufficient Permission"
   }
  ],
  "code": 403,
  "message": "Insufficient Permission"
 }
}

Any idea, why I am receiving this error message?

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