>>>What's UNIQUE?
To be honest, I don't know (and I doubt that anyone else here will be sure of an answer). However I'd guess that you've used your email address somewhere else with the same website (or with a different website which shares addresses with the one which you're trying to register with) and that you're not allowed to use it again.
I've occasionally come across websites which won't allow you to use email addresses from providers other than ISPs. (i.e. they won't accept addresses ending in things like gmail.com or outlook.com but they will accept addresses ending in things like @virginmedia.com or @btinternet.com). However I can't see how their 'approved' addresses could be regarded as 'unique'.
However, if one address doesn't work, simply try another. Your existing email provider probably allows you to create 'aliases' or you could try using an address from a provider such as GMX (which I recommend):
http://www.gmx.co.uk/