How to use the fillna method in a for loop
what you are looking for is replace().

And you don't need to write all the columns you can iterate over the columns name simply.

for col in train:
    train[col].replace("NA","XX",inplace=True)


You can do it on all the dataset in one line:

train.replace("NA","XX", inplace=True)


Or on specific columns:

for cols in na_data:
    train[col].replace("NA","XX",inplace=True)