Thank you for your update, @barwalrules.
The “Disable XML-RPC” option is adding rules to the website main .htaccess file. Usually this error is returned if the plugin can not write to the file to add the rules to it.
I suggest you to double-check the file in question. Make sure it is having correct permissions (644) and it is not locked for writing. In case you are facing difficulties with those checks, I recommend contacting your hosting provider support for assistance on the matter.
Additionally – there should not be any other plugins active with the same functionality enabled on them. Our plugin will display a message to inform you if we have found other plugins with this functionality. We will automatically disable the option from our plugin to avoid issues.
Best Regards,
Elena