Enable post pages greyed out in Blogger

I wanted to make home page posts to show excerpt and the first step towards implementing is to Enable Post page option from Settings->Archiving. However Enable post pages is greyed out.

Enable Post Pages option greyed out

To enable Enable post pages option you have to do followings:
From Settings->Comments->Comment Form Placement
Select Full Page option and save the settings.

Set Full Page

Now go back to Settings->Archiving and now you will see this option is enabled.