When I tried to assign a blog page in WordPress settings (reading) it used the ‘Blog archived’ setting on the theme, what I want is to use the blog page that I created manually.
The blog is an Archive page and is controlled by WordPress, it doesn’t allow any modifications added to it, this is the default functionality. You cannot edit it as well. Please navigate to Theme Settings -> Blog -> Blog archive and check the settings. Also, you can configure the option about Single post.