I am a system admin and not a developer. I need help with test code that is not working.

I downloaded a free App off the AppExchange called AttachmentSaver. (This stops users, except system admins, from deleting Attachments.) The test code coverage is only reaching 50% as the test is failing at line 11. Can anyone tell me what needs to be amended so that it works?

@isTest
private class TestAttachmentSaver {
 
  /**
  * Verify that Standard User profiles are unable to delete attachments
  */
  static testMethod void testStandardUser() {
   
    // Create a new user with the Standard User profile
    Profile standardProf = [select id from profile where name='Standard User'];
    User su = new User(alias = 'standt', email='standarduser@testorg.com',
      emailencodingkey='UTF-8', lastname='Testing', languagelocalekey='en_US',
      localesidkey='en_US', profileid = standardProf.Id,
timezonesidkey='America/Los_Angeles', username='standarduser@testorg.com');
     
    // Switch current user to Standard User
    System.runAs(su) {   
   
      // Create test data (a new Account with an Attachment)
      Account acct = new Account(Name = 'Test Account');
      insert acct;
      Blob attachBody = Blob.valueOf('attachment body');
      Attachment attach = new Attachment(Name = 'TestAttachment', ParentId = acct.Id, Body = attachBody);
      insert attach;
   
      // Verify the "Unable to delete attachments" error is thrown when attachment is deleted
      Boolean errorThrown = false;
      try {
        delete attach;
      } catch (Exception e) {
        System.debug(e);
        if (e.getMessage().contains('Unable to delete attachments.')) {
          errorThrown = true;
        }
      }
      System.assert(errorThrown);
        }
  }
 
  static testMethod void testAdminUser() {
   
    // Next make sure that a System Admin *can* delete an attachment
    Profile adminProf = [select id from profile where name='System Administrator'];
    User au = new User(alias = 'admint', email='adminuser@testorg.com',
      emailencodingkey='UTF-8', lastname='Testing', languagelocalekey='en_US',
      localesidkey='en_US', profileid = adminProf.Id,
      timezonesidkey='America/Los_Angeles', username='adminuser@testorg.com');
   
    // Switch current user to System Admin user
    System.runAs(au) {
     
      // Create test data (a new Account with an Attachment)
      Account acct = new Account(Name = 'Test Account');
      insert acct;
      Blob attachBody = Blob.valueOf('attachment body');
      Attachment attach = new Attachment(Name = 'TestAttachment', ParentId = acct.Id, Body = attachBody);
      insert attach;
     
      // Verify that no error is thrown when the attachment is deleted
      Boolean errorThrown = false;
      try {
        delete attach;
      } catch (Exception e) {
        System.debug(e);
        errorThrown = true;
      }
      System.assert(!errorThrown);
    }
  }
}