This popped for me after only a few games when I had selected some Ops to complete. To choose operations, go to the Ops tab which from the tank Garage view is one
In the Ops screen, you can choose up to 3 Ops to work on at a time, so try to pick Ops you think you will be completing with whichever tank you're currently working on. There are also some hidden ops(like winning 3 games in a row), and bonus Ops which don't count against your 3 choices.
Some easy ops are cumulative XP, surviving and assisting, or even destroy one artillery. There are many to choose from over time, so pick ones you think you can do the easiest!
One last thing--many ops can be completed more than once: when looking at the op, sometimes it will say things like this "Repeatable: Daily (0/3)" which means you can do that particular operation 3 times that day.