An unhandled exception occurred while processing the request

Hi All,
I am getting the below error while implementing the .net web application sample.
Please help me to resolve this error.

Access Denied (authentication_failed)
Your credentials could not be authenticated: “Credentials are missing.”. You will not be permitted access until your credentials can be verified.
This is typically caused by an incorrect username and/or password, but could also be caused by network problems.

IOException: IDX20807: Unable to retrieve document from: ‘’. HttpResponseMessage: 'StatusCode: 407, ReasonPhrase: ‘Proxy Authentication Required’, Version: 1.1, Content: System.Net.Http.HttpConnectionResponseContent, Headers:
Proxy-Authenticate: NEGOTIATE
Proxy-Authenticate: NTLM
Proxy-Authenticate: BASIC realm=“XXX_Windows_Domain”
Cache-Control: no-cache
Pragma: no-cache
X-XSS-Protection: 1
Proxy-Connection: close
Connection: close
Content-Type: text/html; charset=utf-8
Content-Length: 894

Hi, Can you please try to open this URL: in your browser?
If you do not have access to it, you need to have API access management feature enabled in your org. You can open a ticket to for the feature flag issue