Hi, I'm Bernard. I'm happy to help!
Thank you for reaching out. If you're receiving a message that your email "isn't available" during the Extended Security Updates (ESU) enrollment process, it may be because that email address isn't eligible for verification or is already associated with an existing ESU enrollment. If you only have one email address, you can try creating a new Microsoft account with a different email address and then use that account to enroll in the ESU program