If you’re evaluating php variables in a string(interpolating the variables directly in a string), it’s best to use double quotes, this way you don’t have to concatenante with dots.
function get_highest_bid( ) {
global $wpdb;
$postid = get_post_id();
$table = $wpdb->prefix . "jwp_bids";
$highest_bid = $wpdb->get_var(
"Select max(bid_amt)
FROM $table
WHERE post_id ='$postid';"
);
echo $highest_bid;
}