AttributeError: 'CART_RT' object has no attribute 'fit'

This error typically occurs when you are trying to call the `fit()` method on an object that does not have this method defined. In this case, the object in question is a `CART_RT` object, and it appears that this object does not have a `fit()` method defined.

To resolve this error, you will need to either define a `fit()` method for the `CART_RT` object or use a different object that does have a `fit()` method defined. Alternatively, you may be calling the `fit()` method incorrectly, in which case you should check the method signature and syntax to ensure that you are using it correctly.