Setting your region
For this book, you will need to set your region to N. Virginia or Ireland. Lex is currently (April 2018) available in those two regions.
AWS has this concept of regions, which are locations around the world where Amazon have their cloud service centers. Each region is separate from all the others for most applications. It is best practice to deploy services to the regions closest to where they'll be used. If your customers are on the west coast of America then choosing N. California or Oregon would be best, whilst choosing Ireland wouldn't be a great choice. Their data would have to go halfway around the world and back each time they use your product.
One other consideration for the regions is that not every region is equal. Some regions have the larger working capacity, while some don't even have all of the services.