The way the Snowdown Shop works is, you will be unable to purchase any skin from your Snowdown Shop if:
1.) The skin you want to buy is on sale in the normal store
or
2.) The champion for the skin you want to buy is on sale in the normal store (this only applies if you don't own the champion).
Since you don't own Bard, and Bard is currently on sale in the normal store, you are unable to make the purchase. You will continue to encounter this "error" until either you buy Bard himself (RP or IP will work) or you wait until 12/29 when Bard is no longer on sale. Either of those choices will get rid of the "error."