As you may know, Lego take a light hearted look at their range, focusing on kid friendly sets including movies and big brands.
So Lego tanks in their current range does NOT exist, or never likely to in the near or far off future. This includes not only tanks, but army men, vehicles, airplanes and boats – basically all equipment associated with the military.
Build your own
This may not be possible as there are no Lego bricks that allow you to replicate the shape and form of a tank, this is because the tank treads and functioning wheels do not exist and will result in a model that resembles more of a square bulky car.
Are there alternatives?
There are Lego like brands available, you can look at Enlighten Bricks and Sluban for example. And well these models are nowhere near the quality of Lego, but they do have a similar style but don’t match or work in conjunction with real Lego bricks. You are also able to build the sets yourself with dozens of bricks and pieces.