How do I close a presentation created from a template and then make a pdf out of it?

I am creating google slides from google sheets. I've succeeded in creating the slides but now I need to convert them to pdf so I can send them out to different people. From what I read, I am supposed to close the slide presentation before making the pdf. I tried using saveAndClose but I get the error

saveAndClose is not a function

Here is the code.

function generateCerts() {
  var ss = SpreadsheetApp.openById('the spreadsheet');
  var sheet = ss.getSheetByName('Sheet1');
  var data = sheet.getDataRange().getValues();
  var templateId = 'the template';
  var folder = DriveApp.getFolderById('the folder');

  // some other code

  for (var i = 1; i < data.length; i++) {
    var templateCopy = DriveApp.getFileById(templateId).makeCopy(folder);
    var slide = SlidesApp.openById(templateCopy.getId()).getSlides()[0];
    var shapes = slide.getShapes();

    // the slides are created in this loop

    var certificateName = school + "_Certificate";
    templateCopy.setName(certificateName);

    templateCopy.saveAndClose();
    var certBlob = templateCopy.getAs('application/pdf').setName(certificateName + '.pdf');
    folder.createFile(certBlob);
  }
}

I have tried separating them into two functions but what happened was I got several PDF copies of the template I used to create the slides instead of the slides with the data already in them.

Someone please tell me what I did wrong and how am I supposed to do this.

Solution

Modification points:

saveAndClose() is a method of Class Presentation. Ref But, in your script, templateCopy is Class File. I think that this is the reason for your current issue of saveAndClose is not a function. This has also been mentioned in TheMaster's comment. Ref
In your showing script, school is not declared. Please be careful about this.

When these points are reflected in your script, how about the following modification?

Modified script:

function generateCerts() {
  var ss = SpreadsheetApp.openById('the spreadsheet');
  var sheet = ss.getSheetByName('Sheet1');
  var data = sheet.getDataRange().getValues();
  var templateId = 'the template';
  var folder = DriveApp.getFolderById('the folder');
  
  // some other code
  
  for (var i = 1; i < data.length; i++) {
    var templateCopy = DriveApp.getFileById(templateId).makeCopy(folder);
    var s = SlidesApp.openById(templateCopy.getId()); // Modified
    var slide = s.getSlides()[0]; // Modified
    var shapes = slide.getShapes();
    
    // the slides are created in this loop

    var certificateName = school + "_Certificate";
    templateCopy.setName(certificateName);

    s.saveAndClose(); // Modified
    var certBlob = templateCopy.getAs('application/pdf').setName(certificateName + '.pdf');
    folder.createFile(certBlob);
  }
}

In this modification, it supposes that school has already been declared elsewhere. Please be careful about this.

Reference:

saveAndClose() of Class Presentation