Saying someone is "not a cultural fit" is a polite way to say that the person is difficult or impossible to work with.

In the workforce nowadays, you are almost never going to get direct feedback about what you might be doing wrong: you have to read between the lines.

If you have been told by several companies that you are not a cultural fit (and then let go), then you might consider what aspects of your behavior are causing friction.

Think of it this way: due to the high costs of acquiring and training employees, companies hate to let people go, especially new hires. One has to be doing something very wrong to get let go, and employees are usually given many chances to get it together.