how can I cancel an app that I recently order.

You are not speaking to Apple, this is a user to user community. 

We are unable to help with refunds, account issues or purchases  

Try applying for a refund here - Request a refund for apps or content that you bought from Apple – Apple Support

Or Contact Apple Support - Contact - Official Apple Support

I tried to order a mouse on line, but it would not let me. So I called to place the order but now I want to cancel it. Unfortunately, the rep did not give me an order number and I did not get an email confirming the order. Can you please cancel the order and confirm it has been canceled.

You need to contact Apple Support

This is a user to user community, we are not Apple employees so we can’t help with orders or accounts.

You need to contact Apple and should be able to cancel the order for you and confirm it’s been cancelled, but as I’ve said this community can not help.

Thanks