[14-Sep-2022 12:30:05 Asia/Kolkata] PHP Warning:  Undefined array key "pk_i_id" in /home/gg/public_html/oc-content/plugins/osclass_pay/email.php on line 158
[14-Sep-2022 12:30:05 Asia/Kolkata] PHP Warning:  Undefined array key "s_title" in /home/gg/public_html/oc-content/plugins/osclass_pay/email.php on line 161
[14-Sep-2022 12:30:05 Asia/Kolkata] PHP Warning:  Undefined array key "pk_i_id" in /home/gg/public_html/oc-content/plugins/osclass_pay/email.php on line 163
[14-Sep-2022 12:30:05 Asia/Kolkata] PHP Warning:  Undefined array key "pk_i_id" in /home/gg/public_html/oc-content/plugins/osclass_pay/email.php on line 172

When did you get these warnings, how to reproduce it?

@umimo
Put this into email.php on line 157 or so.
  if(!is_array($item) || empty($item) || !isset($item['pk_i_id']) || @$item['pk_i_id'] <= 0) {
    return false;
  }
Error most probably means problem on your site, you are trying to send email for listing that does not exists anymore. (or was not added)