You are trying to register as a New Supplier but cannot as you are seeing the message :
This means your email address is already registered in this AWARD account.
Recommended action is to use the Registered Supplier option (you will see this option when you first click a registration link).*
However, if you still wish to create a new account (in addition to your existing account) you will need to use +addressing
i.e. rather than use Buzz.Aldrin@test.com please use Buzz.Aldrinemail@example.com (the word after the + can be anything) this will give you a unique email address for the new registration.
*if you have forgotten your login details please click here