Thank so much for your answers.
Anyway, is this correct?
$imgcategory = get_field('imagen_de_categoria', $category);
echo '<div class="menu_section_parallax" style="background-image: url( '.$imgcategory. ' )"></div>';
Or this?
echo '<div class="menu_section_parallax" style="background-image: url( '.get_field('imagen_de_categoria', $category). ' )"></div>';
Because the images are not showing