Apex domains cannot have CNAME records.
Question and response
Question
dig sharepointonline.com. NS
Answer
sharepointonline.com. 5 NS ns2-221.azure-dns.net.
sharepointonline.com. 5 NS ns4-221.azure-dns.info.
sharepointonline.com. 5 NS ns1-221.azure-dns.com.
sharepointonline.com. 5 NS ns3-221.azure-dns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0