Your Canvas account utilizes a preferred email to send notifications to and associate with the rest of Florida State's IT resources. In particular, you will need to know your preferred Canvas email if you want to login to Turnitin.com directly.
You do not need to login to Turnitin directly in order to use Turnitin within your Canvas courses, but being able to do so may be helpful if you need to resubmit a paper on your students' behalf.