Why is it showing "The following required properties are missing: fb:app_id"?

If you check your page on Facebook Debugger, it may show " The following required properties are missing: fb:app_id". 

This property is the id of the Facebook app used to identify the developer. In fact, our code snippet has included the property. If you've already installed our code snippet into your theme, the app id is added. 

<meta property="fb:app_id" content="1015003798605559" />

However, Facebook debugger is not responsive to read it. If you press the "Scrape Again" button again, the message will be gone. 

Also, this property will NOT affect Facebook link previews at all. If you have filled in the title, description, and image in SPO, it will be translated into the og tags and put to your home page. The og tags will be read by Facebook when you share the page link. 

So, don't worry about it.