It is so simple, you have 2 objects with the same name and when you say: bank_holiday[month] python thinks you wanna run your function and got ERROR.
Just rename your array to bank_holidays <--- add a 's' at the end! like this:
bank_holidays= [1, 0, 1, 1, 2, 0, 0, 1, 0, 0, 0, 2] #gives the list of bank holidays in each month
def bank_holiday(month):
if month <1 or month > 12:
print("Error: Out of range")
return
print(bank_holidays[month-1],"holiday(s) in this month ")
bank_holiday(int(input("Which month would you like to check out: ")))