The error message indicates that the utility "wkhtmltopdf" was not found at the expected location. This is configured at Administer -> System Settings -> Misc (/civicrm/admin/setting/misc?reset=1), "Path to wkhtmltopdf executable".
You will need to find whether this utility is installed on the new server and if so, enter the full path to the wkhtmltopdf executable. If it's not installed, you can leave this field blank and CiviCRM will use the default PDF generation method instead. However this has performance issues and may cause errors from exceeding PHP memory limit or timeout.
If you don't actually need a PDF copy of receipts, you can disable this with the "Attach PDF copy to receipts" setting on the same form.