Lean and Six Sigma are both excellent methodologies that provide excellent results when used properly.
The problem, in regards to certification, is that no one organization owns/controls the Lean or Six Sigma certifications - unlike the Project Management Professional certification. PMI owns the PMP. PMI controls the body of knowledge tested in the exam, the requirements to be eligible for the certification, and the exam.
I suggest you check this out https://www.edureka.co/six-sigma-green-belt-self-paced
As for career advancement potential, that depends on your organization.
Some will care. Some will not. However, if you can show a pattern of learning and earning certifications - i.e., a dedication to improving your knowledge and skills - then the Six Sigma can fit into and reinforce that pattern, which can make you a more desirable candidate for a job/promotion.