Hide prices from certain category in woocommerce

The is_category() method checks whether the current query is for an existing category archive page. Being woocommerce you should use the is_product_category() method instead, and to check for single product the has_term($slug, 'product_cat', $product_id) method.

This code will remove prices from archive pages and single product pages:

add_action( 'woocommerce_after_shop_loop_item_title', 'hide_loop_product_prices', 1 );
function hide_loop_product_prices(){
    global $product;

    if( is_product_category('sold') ):

    // Hide prices
    remove_action('woocommerce_after_shop_loop_item_title', 'woocommerce_template_loop_price', 10 );
    // Hide add-to-cart button
    remove_action('woocommerce_after_shop_loop_item','woocommerce_template_loop_add_to_cart', 30 );

    endif;
}

// Single product pages
add_action( 'woocommerce_single_product_summary', 'hide_single_product_prices', 1 );
function hide_single_product_prices(){
    global $product;

    if( has_term( 'sold', 'product_cat', $product->get_id() ) ):

    // Hide prices
    remove_action('woocommerce_single_product_summary', 'woocommerce_template_single_price', 10 );

    // Hide add-to-cart button, quantity buttons (and attributes dorpdowns for variable products)
    if( ! $product->is_type('variable') ){
        remove_action('woocommerce_single_product_summary','woocommerce_template_single_add_to_cart', 30 );
    } else {
        remove_action( 'woocommerce_single_variation', 'woocommerce_single_variation_add_to_cart_button', 20 );
    }

    endif;
}

Reference: Hide Price based on product category in Woocommerce