How can I "get" a Google sheet through App Scripts?

I'm trying to connect a Google Sheet to Qualtrics. And keep running into this error: Sheet "Completion_Codes" not found.

Here's the script:

function doGet() {
var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Completion_Codes");
if (!sheet) {
Logger.log('Sheet "Completion_Codes" not found');
throw new Error('Spreadsheet not found');
  }

var data = sheet.getDataRange().getValues();
var unusedCodes = [];

for (var i = 1; i < data.length; i++) {
if (data[i][1] !== 'Yes') {
unusedCodes.push(data[i][0]);
    }
  }

if (unusedCodes.length === 0) {
return ContentService.createTextOutput(JSON.stringify({ "completionCode": "No more codes available" })).setMimeType(ContentService.MimeType.JSON);
  }

var randomIndex = Math.floor(Math.random() * unusedCodes.length);
var selectedCode = unusedCodes[randomIndex];

for (var i = 1; i < data.length; i++) {
if (data[i][0] === selectedCode) {
sheet.getRange(i + 1, 2).setValue('Yes');
break;
    }
  }

return ContentService.createTextOutput(JSON.stringify({ "completionCode": selectedCode })).setMimeType(ContentService.MimeType.JSON);
}

I have verified the name multiple times, even made sure I'm on the "active sheet" but nothing is working atm.

You are getting 'Sheet "Completion_Codes" not found' because the active spreadsheet does not have a sheet named Completion_Codes.

The sheet name parameter is case sensitive.

Something that might help you and others with the same problem is to log the sheet names using something like this:

function logSheetNames(){
  SpreadsheetApp.getActiveSpreadsheet()
     .getSheets()
     .forEach(sheet => Logger.log(`"${sheet.getName()}"`);
}

The above code will add a log entry for each sheet. The sheet names will be quote enclosed.