LearnDash includes the option to reward your users with certificates upon completion of course material such as lessons. To make designing the certificates easier, LearnDash offer the free Certificate Builder Add-On. If you're having trouble getting it to work with the Divi theme, this post will hopefully help you troubleshoot the issue.
Note, see this post if you want to instead design the LearnDash certificate using the Divi Builder.
Disable Classic Editor
A while back, WordPress introduced the "Gutenberg" block editor as a replacement for the old plain text post editor. Since Divi's original page builder (now called the "classic builder") relied on the old post editor, Divi added a feature to restore the classic editor – and in the process disable the WordPress block editor.
However, since the LearnDash Certificate Builder Add-On depends on the block editor, this Divi feature prevents the Add-On from working. To solve this, you can disable the classic editor setting in Divi by going to:
Divi > Theme Options > Builder > Advanced > Enable Classic Editor
And setting it to "Disabled", like so: