Many courses offer certificates, but they also require you to pay for the certificate. At the moment, no major MOOC provider routinely offers a free certificate, although FutureLearn offers a free upgrade on some courses. See 180+ FutureLearn Courses That Still Offer Free Certificates.
Currently, some certificates are available for free for a limited time. Please see https://www.classcentral.com/report/free-online-learning-coronavirus/
Many course providers offer a free trial or a money-back period after payment. You may be able to pay for the certificate after starting the course. Check the course provider's information pages carefully for these details.
If you want a certificate but cannot afford the cost, some providers offer financial aid to needy applicants. See Can I get financial aid for MOOCs?
In addition to individual course certificates, some providers have created their own certificate programs. These programs are something between a single course and a full degree, and they usually involve taking a sequence of courses. Examples of these would be Coursera’s Specializations, edX’s XSeries, or Udacity’s Nanodegrees.