Kadence - How to Add a Sidebar to a Page
All Kadence child themes come with 2 sidebar widget areas by default: Sidebar 1, and Sidebar 2.
You can find these sidebar widget areas in Appearance > Widgets.
In your page settings (either the global page settings or in the individual page settings), you have page layout options. Some of the layout options include a sidebar. If you choose the sidebar option, you will need to choose either Sidebar 1 or Sidebar 2. Depending on which sidebar you chose, you will need to add widgets to that sidebar for it to show up in Appearance > Widgets.
For example, if you set your Contact page to have a right sidebar layout and have chosen Sidebar 1 to display, this means you will need to go to Appearance > Widgets > Sidebar 1 > Add your sidebar widgets here
You can adjust the settings and design of Sidebar 1 and Sidebar 2 by going to Appearance > Customize > General > Sidebars