Let's start with some basics, swipe down from the top right side of the tablet and tap "Settings", then scroll down and tap "Language and Input" and make sure Google Keyboard is selected. You did not mention, if you can installed a 3rd party keyboard, but if so it will be listed. If the "@" returns after selecting the Google keyboard and you had another keyboard selected, you have identified your issue. Personally, I use the "SwiftKey" keyboard for tablets from Google Play, that is a great keyboard alternative.
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