before add to cart hook woocommerce
function so_validate_add_cart_item( $passed, $product_id, $quantity, $variation_id = '', $variations= '' ) {
// do your validation, if not met switch $passed to false
if ( 1 != 2 ){
$passed = false;
wc_add_notice( __( 'You can not do that', 'textdomain' ), 'error' );
}
return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'so_validate_add_cart_item', 10, 5 );