Twitter Friendship/create api asking for authentication in swift



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I am trying post a follow the user but am still not able to hit post api POST

it is showing :

Error: Optional(Error Domain=TwitterAPIErrorDomain Code=220 “Request failed: forbidden (403)” UserInfo={NSLocalizedFailureReason=Twitter API error : Your credentials do not allow access to this resource. (code 220), TWTRNetworkingStatusCode=403, NSErrorFailingURLKey=, NSLocalizedDescription=Request failed: forbidden (403)})
can anyone help me out for this. Thanks in advance