Hello There,
Thanks for writing in! You can set your blog index to fullwidth or inherit the global layout settings. You may go to the customizer, Appearance > Customize > Blog > Layout. If it is already set to “Use Global Content Layout”, it would seem that you have selected a fullwidth site layout in Appearance > Customize > Layout & Design > Content Layout. You need to select either “Content Left, Sidebar Right” or “Sidebar Left, Content Right” to be able to activate the sidebars.
Hope this helps.