The certificate module is not a standard part of moodle. You should be able to find a ZIP file by browsing/searching these forums which contains the files you need.
You should unzip the contents of the Zip package into your moodle folder on your server. Then visit your Moodle administration page and the Certificate module will then install.
Make backups first though in case anything goes wrong.
General plugins
What does the Certificate Module Do?
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