Hi there and welcome
You will need to connect the Kindle to your wifi network
To see the available networks, tap on "Menu" again, now "Settings" and then on "Wi-Fi Networks."
If this is the first time connecting to the local wireless network, the device with take a moment to scan and access the available wireless networks.
Tap on the desired network.
Enter the password ***** once connected you should be able to purchase books again
Let me know the results please