When someone says "Don't bank on it," they are expressing doubt or skepticism about a particular outcome or prediction. It means you should not rely heavily or put a lot of confidence in the possibility of something happening. In other words, the person is cautioning you not to expect or depend on the event or situation to occur because they believe it is unlikely or uncertain.
The phrase "Don't bank on it" is often used to manage expectations and advise against making plans or decisions based on the assumption that something will definitely happen. It suggests that there is a significant chance of disappointment or failure if you rely too much on the anticipated outcome.