How do I handle this work error?

I’m trying to figure out how to explain this work error. I forgot to input an order into our system. I only noticed because the buyer emailed me asking for an ETA as it was due that day. I immediately told him I was looking into it and put the order in. It’s delivering on Thursday.

Now he wants to know where the order is. I’m trying to figure out how to respond. I don’t want to say I forgot to input the order in because that’s a bad look on our company. I was going to say there was a system error and it’s delivering Thursday but that’s sort of a lie.