I echo the advice to start low and increase the power. But also DON’T MOVE YOUR MATERIAL. If you laser it and nothing happens you can adjust your settings and rerun the job. The glowforge will laser again in the same exact place. If the leather is not in the same exact place well, you have a problem. There are ways around this, but for someone very new to the glowforge it is simplest not to move it if the results aren’t what you want. Also, make sure the artboard of your design is 12x20 inches.
Most fashion leather is chrome tanned and not vegetable tanned. While every piece of leather engraves and cuts differently, vegetable tanned leather is more consistent piece to piece than chrome tanned. Also, chrome tanned may be more melty. Just stuff to be aware of.
Double check the bag for a tag that guarantees it is actual leather. While I wouldn’t expect Kate Spade to use a vinyl in place of leather, it is good to be sure.
And finally, the finished work people show off, or sell, from the glowforge is usually the result of lots of trial and error. Having at least one, if not more, sacrificial pieces is pretty common. I don’t know how expensive a Kate Spade bag is, but I’m guessing it could get pretty expensive using them to dial in your settings. At the very least practice engraving on draftboard until you have a basic understanding of the glowforge and a process to make stuff work.