How to display html element and php string in the same line?

You must not use use echo or print in a shortcode function. The echoed string will be printed out immediately when the content is processed, which is almost always too early.

So in your function you should just return the string without using echo:

function swh_woocommerce_store_credit_shortcode() {
    $store_credits = swh_woocommerce_get_store_credits();
    return "<span style="display: inline;">Remaining Credit:</span>" . wc_price($store_credits);    
}