Change user role when user’s first post gets published

There is a syntax error in the add_action(). You’ve used a period . instead of a comma , to separate the action and the callback. Otherwise the callback seems to work as expected when I tested it locally.

The correct syntax would be,

add_action( 'pending_to_publish', 'my_function', 10, 1 );

When doing development work with WordPress it is a good idea to have debugging on. This way you can check the log file for any errors when something is not working as expected. Further reading, Debugging in WordPress.

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P.S. If you want the role change to happen only when your custom post type is published, then add a post type check to the top of the callback. Like so,

function my_function( $post )
{
    if ( 'portfolio' !== $post->post_type ) {
        return;
    }
    
    // code...
}