Allowing a CPT post to be edited by a single user role

Add code – your current active theme’s functions.php file or in a custom plugin:

// Step 1: Create a New Role Based on Subscriber
function create_custom_role() {
    // Check if the role doesn't already exist
    if (!get_role('username-role')) {
        // Get the capabilities of the Subscriber role
        $subscriber_role = get_role('subscriber');
        $capabilities = $subscriber_role->capabilities;

        // Create a new role based on Subscriber
        add_role('username-role', 'Username Role', $capabilities);
    }
}
add_action('init', 'create_custom_role');

// Step 2: Assign the New Role to the User
function assign_custom_role_to_user($user_id) {
    $user = new WP_User($user_id);
    $user->set_role('username-role');
}
add_action('user_register', 'assign_custom_role_to_user');

// Step 3: Grant Edit Post Capability for the CPT
function grant_edit_post_capability($post_id, $post, $update) {
    // Check if it's the desired CPT post type
    if ($post->post_type === 'your_cpt') { 
        // Please replace 'your_cpt' with the actual slug of your custom post type

        // Get the user who created the post
        $user_id = $post->post_author;

        // Get the user's role
        $user = new WP_User($user_id);
        $user->add_cap('edit_post', $post_id);
    }
}
add_action('save_post', 'grant_edit_post_capability', 10, 3);