I have the below series:

my_series = pd.Series([np.nan, np.nan, ['A', 'B']])

I have to loop through my_series and evaluate whether the value is NaN or not and then do something (actions defined as 'do A' and 'do B' for the sake of simplicity).

1st try:

for element in my_series:
    if element.isna():
        print('do A')
    else:
        print('do B')

When running it, I've got the error: "'float' object has no attribute 'isna'"

2nd try from the question: Error: float object has no attribute notnull

for element in my_series:
    np.where(element.isnull(), 'do A', 'do B')

When running it, I've got the error: "AttributeError: 'float' object has no attribute 'isnull'"

I haven't found any other similar question here at StackOverflow, I don't know what else to try.

No need for explicit for loops. Based on your second attempt:

# Setup
my_series = pd.Series([np.nan, np.nan, ['A', 'B']])

# Solution
np.where(my_series.isnull(), 'do A', 'do B')                                              

# Output
array(['do A', 'do A', 'do B'], dtype='<U4')