Setting a Plugin Object
You can't automatically expand variables inside strings in Twig as you would normally do in PHP.

Instead, you have to concatenate your strings.

{% set seo = craft.seo.custom('Discount codes and vouchers for ' ~ query, 'You searched for ' ~ query ~ '. Check out the latest live ' ~ query ~ ' discount codes &amp; voucher codes for ' ~ now|date('m') ~ ' ' ~ now|date('Y') ~ ' only at Vouchertoday.uk') %}