Why does fillna with median on dataframe still leaves Na/NaN in pandas?

The problem is with this line:

TT_df = TT_df.fillna(TT_df.median())

Your dataframe has strings and you are attempting to calculate medians on strings. This doesn't work.

Here's a minimal example:

import pandas as pd, numpy as np

df = pd.DataFrame({'A': ['A', 'B', np.nan, 'B']})

df = df.fillna(df.median())

print(df)

     A
0    A
1    B
2  NaN
3    B

What you should do is fillna with median only for numeric columns:

for col in df.select_dtypes(include=np.number):
    df[col] = df[col].fillna(df[col].median())