How would you explain this screw up at work?

I get orders to put into a system but I missed one. I sushi notice until the buyer asked for an ETA. I got the order put in and pushed out early but realized I forgot to explain the error to the buyer.

How do I explain it? I prefer to go vague and not say I forgot to input it. Would saying there’s an error that caused the supplier to not receive the order ok?