Button

you can see that example on nearly most of enfold pages that have longer content: the scroll-top link is a button that is in fixed positon.
Fixed positioning needs position values ( top, bottom, left or right)

so first give a custom-class or unique ID to that button element: in my case it is ID: fixed-button
f.e. to quick css:

#fixed-button.avia-button-wrap {
    position: fixed;
    left: 50px;
    bottom: 50px;
    z-index: 1050;
}

but my advice would be to place a button via button shortcode outside wrap_all container by child-theme functions.php.
There is a hook : ava_after_body_opening_tag that can be used for it.
( this is site-specific only for my testpage – adjust page-id to your needs):

add_action('ava_after_body_opening_tag', function() {
    if(is_page(457)){
      echo do_shortcode("[av_button label='fixed-button' icon_select='yes' icon='ue800' font='entypo-fontello' link='manually,http://' link_target='' size='large' position='center' label_display='' title_attr='' color_options='color_options_advanced' color='theme-color' custom_bg='#444444' custom_font='#ffffff' btn_color_bg='aqua' btn_custom_bg='#444444' btn_color_bg_hover='blue' btn_custom_bg_hover='#444444' btn_color_font='custom' btn_custom_font='#ffffff' id='fixed-button' custom_class='' av_uid='av-khd9fkov' admin_preview_bg='']");
    }
});

see here f.e.: https://webers-testseite.de/impressum/ left bottom corner