add_action( 'rest_api_init' , 'wt_rest_api');
function wt_rest_api(){
   register_rest_route('wtrest','products',array(
           'methods'   => WP_REST_SERVER::READABLE,
           'callback'  => 'wtProductResults'
       ));
}

function wtProductResults($data){
    $products = new WP_Query([
           'post_type' => 'product',
           'tax_query' => array(
                               array(
                                   'taxonomy' => 'product_cat',
                                   'field' => 'term_id', //can be set to ID
                                   'terms' => $data['cat'] //if field is ID you can reference by cat/term number
                               )
                           )
        ]);

    $productsResults = [];
    global $woocommerce;
    global $product;
   $currency = get_woocommerce_currency_symbol();

    while($products->have_posts()){
        $products->the_post();
        $product_cat = get_term(  $data['cat'], 'product_cat', 'category', "OBJECT" );
        $regularPrice = get_post_meta( get_the_ID(), '_regular_price', true);
        $sale = get_post_meta( get_the_ID(), '_sale_price', true);
        $price = get_post_meta( get_the_ID(), '_price', true );
        array_push($productsResults , [
               'title' => get_the_title(),
               'productId' => get_the_id(),
               'permalink' => get_the_permalink(),
               'thumbnail' => get_the_post_thumbnail(),
               'excerpt' => get_the_excerpt(),
               'regularPrice' => $regularPrice,
               'price'     => $price,
               'salePrice' => $sale,
               'category' => $product_cat->name,
               'isVariable' => is_type('variable'),
               'variationPrice' => get_variation_prices()//**Here is My problem**
            ]);
    }
    wp_reset_postdata();
    return $productsResults;

}