Hello @J_Andersn,
Thanks for writing in!
Please be informed that a post template will follow the global content layout settings. If you want to have a sidebar in your posts, please go to X > Theme Options > Layout & Design > Content Layout and select “Content Left, Sidebar Right”. You should be able to add the sidebar widgets for the main sidebar in Appearance > Widgets.
Once you have selected the “Content Left, Sidebar Right” as your global content layout, all your pages will have a sidebar too. Now if you want to have a fullwidth page different from your global settings, you will have to edit of your pages and choose “Layout - Fullwidth” as you page template. To know more about page templates in the theme, please check this out: https://theme.co/apex/forum/t/features-page-templates/50/1
Hope this helps.