Please do as the developer suggests and do a manual FTP delete. Please do this first.
"You will need to manually delete the plugin via ftp. It can be found within /wp-content/plugins/ms-robotstxt-manager."
I found this after deactivating all my plugins, setting up a multisite by turning on networking. Tools> Networking, I had to change the .htcaccess file and the wp-config.php files. This all took about 30 minutes. It took 5 minutes to undo it. I had commented out what they wanted me to replace and kept a back up copy of the files just in case.
It took me 5 minutes to delete the plugin folder, edit the 2 files back to their original state, log out and log back in, reactivate my plugins, and fixed. Plugin is now gone.