desolutionz

You can do this by visiting your hosting Cpanel (Control panel). When you login to your Cpanel, look at the domain section, there you will see Addons domain, subdomain and other domain tools. Click on subdomain, on the list of all your subdomains, look for the one you wish to delete, then delete it from there.
But mind you, when you delete a domain or a subdomain, the directory created when you were adding or creating the domain or subdomain, will not be deleted together. So therefore, the directory will still be in your cpanel file manager. So you have to visit your cpanel file manager and the delete the directory bearing the name of the domain or subdomain you deleted initially.

I am not that too good with the plugin in question. But i think the best you should is to contact the plugin management. I think with them, you will be able to get setp by steps on how to get this error fixed.

Some months back, i uploaded a plugin to my site and errors similar to this came up, so i have to email the management and i they were able to guide me on what to do to resolve it.