Your example didn't state exactly where you wanted this. I've added it to the cart page via the woocommerce_before_cart_table action hook. Which would look like this after adding some extra CSS styling.

You could also add this to a template file of course without the add_action/function mark up.
Action hook:
add_action( 'woocommerce_before_cart_table', 'cart_discount_progress_bar' );
function cart_discount_progress_bar() {
// Create progress bar
$cart_total = WC()->cart->get_cart_contents_total() + WC()->cart->get_cart_contents_tax();
printf( '<progress id="cart-progress-bar" max="100" value="%s"></progress>', $cart_total );
// Create steps
$steps = array(
0 => '',
30 => __( '3% discount', 'discount-bar'),
50 => __( '8% discount', 'discount-bar'),
100 => __( '10% discount', 'discount-bar'),
);
echo '<div id="cart-progress-steps">';
foreach ( $steps as $step => $discount ) {
printf('<div class="step step-%1$s" style="left:%1$s%%">%2$s<span class="discount">%3$s</span></div>', $step, wc_price( $step ), $discount );
}
echo '</div>';
}
CSS:
.woocommerce-cart #cart-progress-bar {
border-radius:0;
border: 1px solid #eaeaea;
margin-top: 1em;
color: cornflowerblue;
background: floralwhite;
}
.woocommerce-cart #cart-progress-bar::-webkit-progress-value {
background: cornflowerblue;
}
.woocommerce-cart #cart-progress-bar::-webkit-progress-bar {
background: floralwhite;
}
.woocommerce-cart #cart-progress-steps {
margin-bottom: 3em;
}
.woocommerce-cart #cart-progress-bar,
.woocommerce-cart #cart-progress-steps {
width: 100%;
position: relative;
height: 20px;
box-sizing: border-box;
}
.woocommerce-cart #cart-progress-steps .step {
position: absolute;
width: 70px;
height: 20px;
font-size: 14px;
text-align: center;
transform: translateX(-50%);
}
.woocommerce-cart #cart-progress-steps .step .discount {
font-size: 12px;
line-height: 11px;
display: block;
}
.woocommerce-cart #cart-progress-steps .step:before {
content: '';
display: block;
position: absolute;
width: 40px;
height: 18px;
border-left: 2px solid black;
left: 50%;
top: -26px;
}
The mistake in your example was that you were trying to use PHP to create a value for a JavaScript variable. So to make that work you would either have to jump in and out of PHP:
var carttotal = <?php echo WC()->cart->get_cart_contents_total() + WC()->cart->get_cart_contents_tax(); ?>;
Or choose to build your whole progress bar in PHP like in my example.