Structured Data Testing Tool Error

Hi, I have the og:type set for Article, since it is a blog post and noticed a structured data error in the webmaster tools:
publisher
A value for the publisher field is required.
It requires to add: itemprop=”publisher” itemscope itemtype=”https://schema.org/Organization”
I am not sure where to place this markup?
It is added to the home page, but not to the blog posts.

Another warning is: mainEntityOfPage.
The mainEntityOfPage field is recommended. Please provide a value if available.
Any suggestion?

Thank you
Andrea

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