Error: “If your robots.txt were writeable”
The Yoast SEO plugin (and thus your entire WordPress install) is not allowed to write to your robots.txt file.
Please make sure that the file is writeable by checking it over FTP. If the robots.txt file is only writeable to the owner and you still get that error, you might need to change permissions on the file.
If you don’t know how to change permissions, please contact your web host for assistance.