I use the above blog as a sandbox
I'm sorry but when you create a thread we Volunteers cannot see the URL you select. Is it the blog linked to your username? If it's another blog please post an active link to it starting with http://
If you are referring to a front page or main page for posts 404 (page not found) caused by deleting the Hello World paceholder post then that error message will be gone immediately after you publish a post, not a page.
Pages are static and are for content that rarely changes. They sit outside of the blog structure and you cannot post to multiple pages in any blog. There is only one main page for posts in a blog and all posts will display on the main blog page. There is no way to exclude posts from the main posts (blog) page.
The front page of the blog by default will display your posts (not pages) in reverse chronological order, with the most recently published post on top. If you do not want all the posts to show on the front page, then you can create a static front page "Welcome" for your site and a “Blog” page for posts.
Choice 1 static front page
Choice 2 running page for all posts on front page
Which do you choose? It’s one or the other.
If you choose Choice 1 static front page then you have to create two pages as I said above. After you do that you go to > Settings > Reading and make the designation change and click “save changes”.