I think it is actually registered.
I did copy and paste your code to have a try on my 5.3.2 installation.
You can try comment out the following to try.
// 'show_in_menu' => 'b2bking',
And then you will discover that it is on your left side menu, probably at the bottom.
Because you set position as 100.
show_in_menu menu means, it will appear under a specific submenu with slug called “b2bking”.
But if that menu slug does not exist, then it will not show. However, it is registered.
You could also test if it is registered by going to the link.
eg. “http://your-site/wp-admin/edit.php?post_type=b2bking_conversation“
If it is registered, you could see the post list screen and add new post for this post type.
Additional notes:
There is one more thing I have changed. Because I test it in functions.php
so I just change the add_action to
add_action( 'init', 'your_function' );
In this way, could reduce other problem like your plugin is activated or not.
Or the function is called or not.