Q: Magic mouse serial number when entered in check warranty gives error as incorrect serial no

The serial number of your Magic Mouse 2 is on the bottom surface of the device, along with other markings. It's a string of 17 letters and numbers.

Identify your Apple wireless mouse, keyboard, or trackpad

Were you checking the warranty because something was wrong.   If so can you tell us the problem.  

Insofar as warranty is concerned, as far as I know, Apple no longer require the serial numbers for warranty purposes on small peripheral items.   Your proof of purchase and establishment of the warranty is your receipt.   Did you buy it from an Apple store?