You can add a sidebar to any page by going to the page settings. You can also add columns.
Go to the Pages tab.
Click on the Settings (gear) icon.
Next to Content section, indicate whether you want to add a sidebar to this page.
Click on Save to add the sidebar, or click on Cancel if you don't want to apply the changes.
⚠ Note: Sidebars can only be used with certain Designs.