Why am I getting the following error in the Fortnite Web Item Shop: "We were unable to complete the transaction, please try again."

If you encountered this error while attempting to purchase an item in the Fortnite Web Item Shop, it means that the item has already left the current Item Shop rotation and can no longer be purchased at this time. Please try to refresh the page to see the currently available items. If the page does not update with new items, visit it later or try the in-game shop. 

For more information related to Item Shop rotation, please see the article: When will items return to the Fortnite shop?