Apex domains cannot have CNAME records.
Question and response
Question
dig shopback.com.br. NS
Answer
shopback.com.br. 5 NS rocco.ns.cloudflare.com.
shopback.com.br. 5 NS harleigh.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0