Apex domains cannot have CNAME records.
Question and response
Question
dig _spf0000006.tangocard.com. NS
Answer
_spf0000006.tangocard.com. 5 NS ns-1906.awsdns-46.co.uk.
_spf0000006.tangocard.com. 5 NS ns-624.awsdns-14.net.
_spf0000006.tangocard.com. 5 NS ns-257.awsdns-32.com.
_spf0000006.tangocard.com. 5 NS ns-1070.awsdns-05.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0