You are not authorized to perform this operation

You are not authorized to perform this operation

I am trying to create an invoice from postman by following steps mentioned in postman collection. 

But I got struck with the following message 

    "code": 57,
    "message": "You are not authorized to perform this operation"

I have entered organization id & oauth token but still getting same issue.

Created customer and item and entered same details in input JSON.

Can any body help me to resolve the issue .

Thanks in advance

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