How do I allow my users to post articles on my site in front-end?

To allow your users to post articles in front-end, you have to use a plugin which provides this feature.
There are several that will allow you to allow your users to create posts, attach the featured image, set categories/tags (and more) in a front-end form directly from their BuddyPress profile.

Social Articles plugin

Social Articles is a basic but very easy to use plugin. This plugin is fully compatible with our themes and plugins.

For more information on how to use this plugin, you can go to their documentation.

BuddyPress User Blog

BuddyPress User Blog is a paid plugin but it’s by far the most powerful by far.
This plugin is not yet compatible with our themes and plugins, but we are working on it.

For more information on how to use this plugin, you can go to their documentation.

Buddyblog

The Buddyblog plugin is also a powerful one and it’s totally free! It’s probably more complicated to set up, though.
This plugin is not yet compatible with our themes and plugins, but we are working on it.

For more information on how to use this plugin, you can go to their documentation.

My users cannot create posts in front-end even after plugin is activated

To allow your users to create/edit/delete posts with WordPress, they must first have the rights to do so.
You can visit this other article of our documentation: How do I allow users of my site to post articles in front-end?

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We will be happy to help you!