'Functional' object has no attribute 'add'

This error message typically occurs when you are trying to call the `add` method on an object that does not have this method defined.

One common cause of this error is when you are trying to add an element to an object that is not a collection or does not support adding elements. For example, if you try to call `add` on an integer or a string object in Python, you will get this error.

Another possible cause is that you have defined a custom object that does not have an `add` method defined. In this case, you will need to define the `add` method in your object to make it work.

To troubleshoot this error, check the type of the object you are trying to add to, and verify that it supports the `add` method. Also, check if you have defined the `add` method in your custom object if you are using one.