Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000003.tangocard.com. NS
Answer
_spf0000003.tangocard.com. 5 NS ns-898.awsdns-48.net.
_spf0000003.tangocard.com. 5 NS ns-1522.awsdns-62.org.
_spf0000003.tangocard.com. 5 NS ns-1634.awsdns-12.co.uk.
_spf0000003.tangocard.com. 5 NS ns-277.awsdns-34.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0