Is there any free online certification course on graphic design?

Unfortunately only certificates that are got from well established institutions have some value as a trustable piece of paper. Others are only papers or even worse: hoaxes. A well established institution can't exist without having a serious mass of already done trustable high quality public work.

I do not believe that there exist established institutons that write to you the certificate that you need. To save their trustability they select carefully who will get a certificate. That takes time because those institutions are schools that teach something and the certificate is given to those that have showed they have the wanted abilities and skills.

There exist no such single test that proves you are a talented and skilled artist. One can get his driver's license very soon, because this is mainly mechanics and the ability to obey the rules. Art is not rules. You must go into a school and do the numerous studies and tests to get your certificate.

Of course you could turn to a not so established institution. In the web you find easily twilight men who write to you just the certificate that you want or let you choose from their vast collection. One time payment - maybe $100,- and you will get a paper that looks out absolutely perfect. People who take this step usually think about some kind of official job that needs the certificate as a primary stipulation to get in. They plan to cover the missing knowledge by behaving smartly and by manipulating the dumb people.

Do not take it! Sooner or later you will be caught. It can take a long time, if you're talented and skilled, but finally happens something unthought and the truth is revealed. That can be an envious person who starts to work systematically to find something useful or simply you have never heard of some person that you should know well. Also you will generate some slight but still noticeable behavioural patterns to keep the hoax hidden. Your house of cards collapses very soon after a single card gets a small thump. After you are stamped to be a mountebank, you're done.

You should let your work talk. If it's extraordinary then some, who already has a name, maybe wants to help you by giving to you some raise in his circle of influence. That way the word spreads. Of course you must try to find those people. No shortcuts are needed assuming you're talented, skilled and a reliable worker, too.