Google API Executable Authentication Error 401 - authentication

Question

This is my first time executing a Google Script as an API executable. I am trying to run it from an Amazon Web-Services micro-server within a HTML file on my server directory. Basically the script is as follows:
function doGet(e){
return HtmlService.createHtmlOutput("Hello World!");
};
To do this, I made use of the JavaScript QuickStart tutorial offered by Google here:
https://developers.google.com/apps-script/guides/rest/quickstart/js#step_2_set_up_the_sample
The thing is that whenever I run the executable, I get an error of code 401 telling me that my script is unauthenticated. The error looks like this:
Error calling API:
{
"error": {
"code": 401,
"message": "ScriptError",
"status": "UNAUTHENTICATED",
"details": [
{
"#type": "type.googleapis.com/google.apps.script.v1.ExecutionError",
"errorMessage": "Authorization is required to perform that action.",
"errorType": "ScriptError"
}
]
}
}
I've tried the following quick-fixes:
Adding the credentials into my Google API Manager with reference to my domain.
Setting the latest deployment of my API executable to be accessible by anyone.
Adding ports to the domain I am running the script from via the API Manager credentials, though it shouldn't be necessary. (I've tested ports 8000, 8080 & 80)
Note also, the domain I am accessing the executable from is given as a plain IP (e.g. XXX.XXX.XXX.XXX). Not sure whether this affects the issue, however I thought it'd be worth mentioning if the error concerns my credential set-up.