How to fix "AttributeError: 'Series' object has no attribute 'codes'" using pandas.Categorical

I am trying to convert a string that is a categorical data type into a numeric. I found out that I can use pandas.Categorical, unfortunately, accessing the codes attribute give me an error.

Here is a minimal example of my working code

>>> sessions_df = pd.read_csv("fitness_sessions_2025.csv")
>>> session_df.head()
  user_name sex age experience_level
0     Alice   F  29     Intermediate
1     Alice   F  29     Intermediate
2     Alice   F  29     Intermediate

>>> sessions_df["experience_level"].unique()
array(['Intermediate', 'Beginner', 'Advanced'], dtype=object)

>>> sessions_df["experience_level"] = pd.Categorical(
...     sessions_df["experience_level"],
...     categories=['Beginner', 'Intermediate', 'Advanced'],
...     ordered=True)

>>> sessions_df["experience_level"].codes
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
~\AppData\Local\Temp\ipykernel_24656\2056368924.py in <module>
----> 1 sessions_df["experience_level"].codes

~\AppData\Roaming\Python\Python39\site-packages\pandas\core\generic.py in __getattr__(self, name)
   6202         ):
   6203             return self[name]
-> 6204         return object.__getattribute__(self, name)
   6205 
   6206     @final

AttributeError: 'Series' object has no attribute 'codes'

Can anyone please explain what I am doing wrong and advise the best approach?