Unless it’s required by law at the area you reside at, it’s not necessary to have one. I train people for endurance events (including marathons and longer distances), but I don’t need a running coach certification for that. If you have the experience and feel comfortable coaching others to be runners, then you should be able to do it with out any extra certification.
I have an RRCA (Road Runners Club of America) Running Coach Certification. While I’ve been running over 28 years, and consider myself to be experienced, I’ve found the information I learned in my certification course to be very useful in developing sound programs for clients. I learned things that I actually didn’t know even with my years of running. Not required but it has definitely been worth having.