By default, comments are enabled on posts however it is possible they are disabled in the Divi Theme Options or the Screen Options in your WordPress Dashboard. 

Make sure they are enabled by going to Divi Theme Options > Layout > Single Post Layout > Show comments on posts.

Last thing to check is that you have Discussion and Allow Comments in your WordPress Dashboard Screen Options

Discussion will then appear at the bottom of your post editor page in your WordPress Dashboard.

If the comments still don't show up, you can also go to Settings > Discussion and make sure the "Automatically close comments on articles older than X days" is disabled.

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