Hello,
You have disabled it in the Theme Options http://prntscr.com/m81x1s
If you want to disable sidebar for the blog page only do this in the blog page options http://prntscr.com/m81yin and enable sidebar in global Theme Options.
Also, you may keep sidebar disabled in global Theme Options but enable it for the necessary pages in page options.
Choose the most suitable for you variant.
Regards