“Item missing. See https://steamerrors.com/26”
According to the link it means that the item doesn’t exist or the account holding the item was banned or something…
How can I get my item/refund?
Access has been revoked (used for revoked guest passes)
You may receive this result when sending, accepting, declining, or canceling a trade offer. Below is an explanation of what this result means in this context.
This response code suggests that one or more of the items in this trade offer does not exist in the inventory from which it was requested."