Structured data mainContentOfPage is not recognized by Google

Hi
I’m using Enfold v3.4.3 just recently created a site.
I’ve seen a number of questions about structured data errors coming up in the Google Webmaster Tools but can’t find the answer to my particular problem.
I was able to fix the problem I had on my portfolio pages with contentUrl by following advice your support gave for the funtions file. But now I still have the following error showing up:

The property mainContentOfPage is not recognized by Google for an object of type MediaObject.
This is only occurring on the portfolio pages.

(I’m not using a child theme yet – I’ve had an issue with the menu items displaying in a different colour and not the same when I tried to change to a child theme). I will post a separate question for that issue. However, I’ve had to make changes to the original functions files for the previous structured data error. I know this is going to be an issue whenever I update the theme as I may not remember to make the changes. So I need to get the child theme sort too.

Thanks for your help!