Apps Script Error - Cannot find method getRange(number,number,(class),number) - google-apps-script

Question

I've written a custom Google Apps Script that will pull some data (2 columns wide, 50-100 rows long but this varies)in an array 2 from an API, parse it into JSON and then paste into a google sheet.
I can run the script from the editor and it works ok. But when I try to run it from a custom menu or when I run the debugger I get the following error:
'Cannot find method getRange(number,number,(class),number) (line 43)'
Line 43 is the last line of the code.
sheet.getRange(3,1,dataSet.length,2).setValues(rows);
It seems that the issue is that getRange is not able to use the variable of length of the dataset (number of rows) to set the number of rows to use in the range in which the data is to be pasted.
I cannot work out how to fix this - can anyone else see what I am doing wrong? Thanks for taking a look.
//custom menu
function onOpen() {
var ui = SpreadsheetApp.getUi();
ui.createMenu('XXXX Data')
.addItem('Credit Limits','CREDITLIMITS')
.addToUi();
}
function CREDITLIMITS() {
var ss = SpreadsheetApp.getActiveSpreadsheet(); //get active spreadsheet
var sheet = ss.getActiveSheet();
// var sheet = ss.getSheetByName('data'); //get sheet by name from active spreadsheet
// URL and params for the API
var USERNAME = 'XXXXXXX';
var PASSWORD = 'XXXXXXXXXXXXX';
var url = 'https://api.XXXX.com/api/v1/XXX/?where=type=%27XXXXXXX%27'; // var url="http://example.com/feeds?type=json"; // Paste your JSON URL here
var authHeader = 'Basic ' + Utilities.base64Encode(USERNAME + ':' + PASSWORD);
var params = {'method': 'GET','muteHttpExceptions': true,'headers': {'Authorization': authHeader,} };
//call the XXXX API
var response = UrlFetchApp.fetch(url, params); // get api endpoint
var json = response.getContentText(); // get the response content as text
var dataAll = JSON.parse(json); //parse text into json
var dataSet = dataAll;
//create empty array to hold data points
var rows=[],
data;
//loop over the retrun events
for (i=0; i < dataSet.length; i++) {
data = dataSet[i];
//push a row of data as 2d array
rows.push([data.company, data.creditLimit]);
}
// clear any previous content
sheet.getRange(1,1,500,10).clearContent();
// write data to sheet
sheet.getRange(3,1,dataSet.length,2).setValues(rows);
}