i dont understand why my if cycle does not work correctly

I wrote next cycle, but when I run it, Spyder shows me next message: ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all(). why it happens? because I specified the value to be assigned to the column if none of the conditions is met.

if 11 >= df['age'] <= 20:
    df['age_enc'] = 20
elif 21 >= df['age'] <= 25:
    df['age_enc'] = 25
elif 26 >= df['age'] <= 30:
    df['age_enc'] = 30
elif 31 >= df['age'] <= 35:
    df['age_enc'] = 35       
elif 36 >= df['age'] <= 40:
    df['age_enc'] = 40    
elif 41 >= df['age'] <= 50:
    df['age_enc'] = 50  
elif 51 >= df['age'] <= 60:
    df['age_enc'] = 60  
else:
    df['age_enc'] = 100;