Hi there,
It should be possible but I’m not sure it fits the overall look and feel of that pre-made site.
But if you must still enable the sidebar, you can try enabling it on Appearance > Customize > Layout > Sidebar and make sure the dropdown options are set to Content / Sidebar
.
If this doesn’t enable the sidebars on the side, you’ll have to check on Appearance > Elements for Layout Elements that modify the sidebar layout as shown here – https://docs.generatepress.com/article/layout-element-overview/#sidebar
Or check the page’s Layout Metabox. https://docs.generatepress.com/article/layout-metabox-overview/