Apex domains cannot have CNAME records.
Question and response
Question
dig medusa.servidoraweb.net. NS
Answer
medusa.servidoraweb.net. 5 NS ns2.servidoreswebs.com.
medusa.servidoraweb.net. 5 NS ns1.servidoreswebs.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0