Reward codes will only work on normal web shop orders, subscription instalments work on a slightly different system so they aren't transferrable. If you've earned a reward that you'd like to use then you'll want to pick some coffees that you fancy the sound of from the online shop and put through a normal order for those - you'll be able to pop the discount code in during checkout to apply your reward to the order :)
If the extra order leaves you overstocked in the kitchen then you can skip upcoming subscription instalments directly via your account while you're enjoying the other coffees :)
There are step by step instructions on our Help Centre here: Can I skip an upcoming subscription delivery?