Apex domains cannot have CNAME records.
Question and response
Question
dig psm.knowbe4.com. NS
Answer
psm.knowbe4.com. 5 NS ns-815.awsdns-37.net.
psm.knowbe4.com. 5 NS ns-1560.awsdns-03.co.uk.
psm.knowbe4.com. 5 NS ns-457.awsdns-57.com.
psm.knowbe4.com. 5 NS ns-1383.awsdns-44.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0