Reason:

No posts or no pages are set as [Homepage] on Reading Settings.

How to fix it: 

Step 1.

Make sure one or more posts or pages are published.

Step 2. 

Make sure "Your homepage displays" are selected as "Your latest posts". Or "A static page" is selected then both of [Homepage] and [Posts page] are chosen different page.

"Your latest posts" is selected

"A static page" is selected.

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