Your line managers email address must be in the same domain as your email address.
Line Manager: firstname.lastname@example.org
In this example, the domain name is buttleuk.org and both email address share the same domain. You can register.
Line Manager: email@example.com
In this example, the line manager email is a different domain and does not match. You will be unable to register.
Please also check that both email addresses and domain names are spelt correctly in all fields. Do not copy and paste email addresses as this can sometimes add an extra space character in the field that will mean the domain does not match.