Comment on WordPress SEO Meta Tags 2016 by SEO Dave.
In other themes you could temporarily add the entire Google Verification Meta Tag as a Text widget via Appearance >> Widgets and it should work. Text widgets aren’t designed for meta tags, but add it and see if it works, when it’s activated delete the text widget.
If it doesn’t work edit the themes header.php file and add the entire meta tag above the body tag. After you’ve verified remove the meta tag (don’t need to keep it after verification).