Access Spreadsheet Cell values from library script - google-apps-script

Question

Is there a way I can access data, stored within a spreadsheet-file from the library script?
I want to use 1 Google Apps Script from multiple Google Spreadsheet files within my Google Drive.
I followed this answer: "you could use Libraries. The idea is that you create one script that you use as a library" and could successfully import the library to my project.
In order to work, the scripts within the library need some of the cell-values stored in the google sheet files. I know how to access the script via a helper function in my sheet-bound script file. For example:
function loc_my_credits()
{
SISTRIXAbfrageFreigabe.my_credits();
}
Whilst "SISTRIXAbfrageFreigabe" is the library name, and my_credits a function within the library.
When I call loc_my_credits from sheets, nothing happens. My best guess: the script cant read data from the spreadsheet file it needs to execute.
The my_credits script from the library file looks like this:
function my_credits(){
// Base URL to access customsearch
var urlTemplate = "https://api.sistrix.com/credits?api_key=%KEY%";
// initialize sheets: 1. Get the spreadsheet, 2. Get the first and the second sheets in this spreadsheet
var spreadSheet = SpreadsheetApp.getActiveSpreadsheet();
var inputSheet = spreadSheet.getSheets()[0];
// Script-specific credentials & search engine
var sistrix_Apikey = inputSheet.getRange('A2').getValue();
var url = urlTemplate.replace("%KEY%", encodeURIComponent(sistrix_Apikey));
var params = {
muteHttpExceptions: true
};
var xml = UrlFetchApp.fetch(url).getContentText();
var document = XmlService.parse(xml);
var root = document.getRootElement();
var items = document.getRootElement().getChildren();
for (var i = 0; i < items.length; i++) {
if(items[i].getName() == 'answer'){
var answer = items[i].getChildren();
return answer[0].getAttribute('value').getValue();
}
}
return 0;
}
Is there a way I can access the data stored in the spreadsheet file from the library script?

Answer 1

I'm writing this answer as a community wiki, since the issue was resolved from the comments section, in order to provide a proper response to the question.
The problem was related to the usage of methods that need scopes that require authorization, therefore it would be expected that simple triggers would show the error:
You do not have permission to call SpreadsheetApp.openById
Google's documentation states that installable triggers would solve the problem:
Installable triggers, however, offer more flexibility than simple triggers: they can call services that require authorization
According to s.Panse, the usage of installable triggers has resolved the issue in this case.
References:
google script openById : You do not have permission to perform that action
Installable Triggers