Apex domains cannot have CNAME records.
Question and response
Question
dig mercateo.com. NS
Answer
mercateo.com. 5 NS ns-240.awsdns-30.com.
mercateo.com. 5 NS ns-1665.awsdns-16.co.uk.
mercateo.com. 5 NS ns-1483.awsdns-57.org.
mercateo.com. 5 NS ns-1005.awsdns-61.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0