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.
BuddyPress User Blog
BuddyPress User Blog is a paid plugin but it’s by far the most powerful by far.
This premium plugin allows your members to create and manage their own posts from their BuddyPress profile. They can set the title, content, categories, tags, featured image and more. As an administrator, you have control over which fields your members are allowed to fill and whether or not posts have to be reviewed by an admin before being made public.
If you want to build a community site and give your users the opportunity to blog, this plugin is exactly what you need.
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?