Apex domains cannot have CNAME records.
Question and response
Question
dig grserver.gr. NS
Answer
grserver.gr. 5 NS ns-396.awsdns-49.com.
grserver.gr. 5 NS ns-1578.awsdns-05.co.uk.
grserver.gr. 5 NS ns-1301.awsdns-34.org.
grserver.gr. 5 NS ns-944.awsdns-54.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0