The truth value of a Series is ambiguous, while using if / else [duplicate]

This question already has an answer here:

• Pandas conditional creation of a series/dataframe column 8 answers

im trying to add if / else statement depending on the result of the previously added column, but I get the below error:

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

below is my code: (all data is numerical data)

Main_df.loc[:,'X']= ((Main_df['Y'] / Main_df['Z']) - 1 ) * 100

if Main_df['Y'] > Main_df['Z']:
    Main_df['X']= abs(Main_df['X'])
else:
    Main_df['X']= - abs(Main_df['X'])