Do you use Google's Structured Data testing tool for validation?

If so, isn't the tool give you a suggestion on the right why your markup doesn't pass the validation?

You might also not be using the author property inside the right object type. It's impossible to detect the problem without an example of your source code.

You can check it on google structured data tool where you can find errors if any that displayed in that tool.

If you are using WordPress platform, so you can use Schema Pro or WP Review Pro or WP Product Review or All In One Schema.org Rich Snippets or Rich Reviews plugin to solve the error. So presently which platform you are using where you developing rich snippt code?

You can check to google structured data testing tool and you can find some suggestion for appearing red mark issues.