How do I modify the custom footer text and get theme version number to show in wordpress admin?

Try returning your span instead of echoing it.

function modify_admin_footer() {
    return '<span id="footer-thankyou">Developed by <a href="http://www.toolcart.com" target="_blank" rel="noopener noreferrer">Toolcart Theme Version:</a>.</span>' . wp_get_theme()->parent()->Version;
}
add_filter( 'admin_footer_text', 'modify_admin_footer' );