As Lisa said, you don't need certifications to work as a writer. People hire freelance writers based on samples and previously published works. If you have a special area of expertise, they look for work experience and education in that area -- science for example.
Beware of scam writing schools, online writing programs, and online writing communities that offer "certifications" for a fee. These "credentials" are worthless. In fact, listing them on your resume is likely to label you as an amateur and fool! Potential employers might think you are bit of dummy, no disrespect, just passing on information gleaned from people in charge of hirings and firings.
Best of luck to you in your writing endeavors!
(Feel free to list real certifications, like the ability to administer CPR to a passed out person, or license to operate heavy machinery!)