Why wouldn’t M&S let me use my voucher for Christmas food?

It wasn’t written down in the exclusions, but I couldn’t use my voucher on a turkey crownIn October I received an M&S Sparks email with a link to a £5-off voucher for “food to order” costing £25 or more online or in store. Next morning I went to the “Christmas food to order” section to choose a turkey crown. But the voucher did not work. M&S told me it did not include the Christmas food as it never discounts this. But I have a screenshot of