It works, the tags are displayed in the loop, But when entering one of the tags appears the error 404 I have also created a template tag-pizza.php

Here code of the tags

add_action( 'init', 'create_tag_taxonomies', 0 );
function create_tag_taxonomies() 
{
  // Add new taxonomy, NOT hierarchical (like tags)
  $labels = array(
    'name' => _x( 'Tags', 'taxonomy general name' ),
    'singular_name' => _x( 'Tag', 'taxonomy singular name' ),
    'search_items' =>  __( 'Search Tags' ),
    'popular_items' => __( 'Popular Tags' ),
    'all_items' => __( 'All Tags' ),
    'parent_item' => null,
    'parent_item_colon' => null,
    'edit_item' => __( 'Edit Tag' ), 
    'update_item' => __( 'Update Tag' ),
    'add_new_item' => __( 'Add New Tag' ),
    'new_item_name' => __( 'New Tag Name' ),
    'separate_items_with_commas' => __( 'Separate tags with commas' ),
    'add_or_remove_items' => __( 'Add or remove tags' ),
    'choose_from_most_used' => __( 'Choose from the most used tags' ),
    'menu_name' => __( 'Tags' ),
  ); 

  register_taxonomy('tag','pizza',array(
    'hierarchical' => false,
    'labels' => $labels,
    'show_ui' => true,
    'update_count_callback' => '_update_post_term_count',
    'query_var' => true,
    'rewrite' => array( 'slug' => 'tag' ),
     'with_front'    => true
  ));

}

here code of taxonomy principal

function my_taxonomies_productpizza() {
  $labels = array(
    'name'              => _x( 'Generos', 'taxonomy general name' ),
    'singular_name'     => _x( 'Generos', 'taxonomy singular name' ),
    'search_items'      => __( 'Search Product Categories' ),
    'all_items'         => __( 'All Product Categories' ),
    'parent_item'       => __( 'Parent Product Category' ),
    'parent_item_colon' => __( 'Parent Product Category:' ),
    'edit_item'         => __( 'Edit Product Category' ),
    'update_item'       => __( 'Update Product Category' ),
    'add_new_item'      => __( 'Add New Product Category' ),
    'new_item_name'     => __( 'New Product Category' ),
    'menu_name'         => __( 'Generos' ),
  );
  $args = array(
    'labels' => $labels,
    'hierarchical'  => true,
    'public'    => true,
    'query_var'   => 'generos',
    //slug prodotto deve coincidere con il primo parametro dello slug del Custom Post Type correlato
    'rewrite'   =>  array('slug' => 'pizza' ),
    '_builtin'    => false,
  );
  register_taxonomy( 'generos', 'pizza', $args );
}
add_action( 'init', 'my_taxonomies_productpizza', 0 );

It works, the tags are displayed in the loop, But when entering one of the tags appears the error 404 I have also created a template tag-pizza.php

Here code of the tags

add_action( 'init', 'create_tag_taxonomies', 0 );
function create_tag_taxonomies() 
{
  // Add new taxonomy, NOT hierarchical (like tags)
  $labels = array(
    'name' => _x( 'Tags', 'taxonomy general name' ),
    'singular_name' => _x( 'Tag', 'taxonomy singular name' ),
    'search_items' =>  __( 'Search Tags' ),
    'popular_items' => __( 'Popular Tags' ),
    'all_items' => __( 'All Tags' ),
    'parent_item' => null,
    'parent_item_colon' => null,
    'edit_item' => __( 'Edit Tag' ), 
    'update_item' => __( 'Update Tag' ),
    'add_new_item' => __( 'Add New Tag' ),
    'new_item_name' => __( 'New Tag Name' ),
    'separate_items_with_commas' => __( 'Separate tags with commas' ),
    'add_or_remove_items' => __( 'Add or remove tags' ),
    'choose_from_most_used' => __( 'Choose from the most used tags' ),
    'menu_name' => __( 'Tags' ),
  ); 

  register_taxonomy('tag','pizza',array(
    'hierarchical' => false,
    'labels' => $labels,
    'show_ui' => true,
    'update_count_callback' => '_update_post_term_count',
    'query_var' => true,
    'rewrite' => array( 'slug' => 'tag' ),
     'with_front'    => true
  ));

}

here code of taxonomy principal

function my_taxonomies_productpizza() {
  $labels = array(
    'name'              => _x( 'Generos', 'taxonomy general name' ),
    'singular_name'     => _x( 'Generos', 'taxonomy singular name' ),
    'search_items'      => __( 'Search Product Categories' ),
    'all_items'         => __( 'All Product Categories' ),
    'parent_item'       => __( 'Parent Product Category' ),
    'parent_item_colon' => __( 'Parent Product Category:' ),
    'edit_item'         => __( 'Edit Product Category' ),
    'update_item'       => __( 'Update Product Category' ),
    'add_new_item'      => __( 'Add New Product Category' ),
    'new_item_name'     => __( 'New Product Category' ),
    'menu_name'         => __( 'Generos' ),
  );
  $args = array(
    'labels' => $labels,
    'hierarchical'  => true,
    'public'    => true,
    'query_var'   => 'generos',
    //slug prodotto deve coincidere con il primo parametro dello slug del Custom Post Type correlato
    'rewrite'   =>  array('slug' => 'pizza' ),
    '_builtin'    => false,
  );
  register_taxonomy( 'generos', 'pizza', $args );
}
add_action( 'init', 'my_taxonomies_productpizza', 0 );

• loop
• tags
• 404-error

• The built in post_tag taxonomy already uses the URL /tag/ by default. Have you inspected the query and verified that it is for the correct taxonomy? – Milo Commented Apr 17, 2017 at 15:19

2 Answers 2

Try resetting permalink.

Go to Settings -> Permalinks and hit save on that page.

Try if tags are working now.

When using the 'tag' name for your custom taxonomy, do not set the query_var parameter to just true. Use a custom string, for example:

register_taxonomy('tag', 'pizza', array(
        'query_var' => 'p',
        'rewrite'   => array( 'slug' => 'tag' ),
));