If your theme does not support the function of adding a custom sidebar to your page or post, you can easily add this custom sidebar function by installing a WordPress plugin called Simple Page Sidebars. With this WordPress plugin, you can create unlimited custom sidebars for your posts or pages. You can also customize each page or post to have its own unique sidebar content.
Another great thing about this WordPress plugin is that you do not need to modify your theme code to use it, meaning there is no need for you to learn programming to use it. To use the Simple Page Sidebars plugin ,you will only need to go to your plugin in setting, search for the plugin then install and activate it and will work instantly.
This plugin also work with custom fields which is another great feature that you cannot miss. With custom fields, you can expand the feature and function that this sidebar can offer.