I believe the spacer shortcode – [tx_spacer size="16"] is provided by a plugin named TemplatesNext ToolKit and it may be that it is designed to work in the page content area, not the sidebars. I don’t know this for sure so I would recommend asking the TemplatesNext ToolKit plugin support if it can be used like this.
To increase the spacing below each widget in the sidebar, you could do this with some custom CSS like this added to Customize > Additional CSS:
#secondary.widget-area .widget {
margin-bottom: 30px;
}