Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000002.tangocard.com. NS
Answer
_spf0000002.tangocard.com. 5 NS ns-1374.awsdns-43.org.
_spf0000002.tangocard.com. 5 NS ns-591.awsdns-09.net.
_spf0000002.tangocard.com. 5 NS ns-496.awsdns-62.com.
_spf0000002.tangocard.com. 5 NS ns-1908.awsdns-46.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0