If your website won't load after updating to BuddyPress 12.1.1 it is likely a conflict with bbPress or another BuddyPress compatible plugin. Install the BP Classic plugin to fix this issue.

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How do I enable comments on pages?

Go to Settings -> Discussion and make sure Allow people to post comments on new article is checked.

Now on any page click the Screen Options button in the top right corner of the page and check the Discussion option.

Since WordPress 4.3 comments are not enabled on pages by default - to enable them use the following plugin: https://wordpress.org/plugins/no-page-comment/