It is unlawful for any person, either for himself or for any other person, to commence, transact, engage in or carry on any business, trade, profession, calling, occupation or gainful activity in the city without having paid the applicable business operations tax and procured a valid business operations tax certificate. Compliance with such requirements shall not be construed to be a condition precedent to engaging in any business or occupation within the city where the imposition of such a condition precedent would be contrary to law. (Prior code § 23.01.101)