Marketo customer can't authenticate via API

If all of the codes are correct, then call Support.

I feel like there are a lot of missing pieces to this question.  

Additionally, there doesn't seem to be an error code of 401 for REST API that Marketo will return.   You can view all the error codes we've encountered here: http://developers.marketo.com/documentation/rest/error-codes/

I am guessing the error here is that an acess token is not being included with the request.
 

As more information, the error message is "unauthorized" and description is "No client with requested id: CLIENT_ID."

Since other customers have authenticated successfully, I'm not sure that it's the access token but we'll double check. 

Murtza, I will email you the full response.

I think your token has expired.
I read same document first, and met same issue.
 https://community.marketo.com/MarketoArticle?id=kA050000000LJcqCAG
If your access token is getting from admin=>LaunchPoint=>View Details=>Get Token, that token has expiration time.
Don't use that token, and your app need to make "Get Access Token" call before expire.
See "Authentication" document.
http://developers.marketo.com/documentation/rest/authentication/

Takehiro makes a good point. The access token expires, so I would make sure to reauthenticate to figure out the source of the error.

Thanks Takehiro.  To clarify, we're seeing a "No client with client ID" response when making the GET request for the authentication token, as described in:
http://developers.marketo.com/documentation/rest/authentication/

We're sending over the client ID and secret, but not getting an access token in the response.

The client is a Marketo customer with the standard plan.

Can you copy and paste JSON response here from Get Access Token request?

ex. JSON response (normal case)
{     "access_token": "cdf01657-110d-4155-99a7-f986b2ff13a0:int",
      "token_type": "bearer",
      "expires_in": 82313,
      "scope": "apis@acmeinc.com"
}

@doxIQ
I am testing authentication error cases.
And I understood your issue is bad client id case only.

BAD CLIENT ID
<oauth>
<error_description>
No client with requested id: 54006d52-6420-418b-90d0-999999999999
</error_description>
<error>unauthorized</error>
</oauth>

BAD CLIENT SECRET
<oauth>
<error_description>Bad client credentials</error_description>
<error>invalid_client</error>
</oauth>

BAD HOST NAME (call to Endpoint)
{"requestId":"9999#999999999999","success":false,"errors":[{"code":"600","message":"Access token not specified"}]}

But you succeeded "quick start guide" procedure.
Are you useing same client ID? or check client ID again.