Apex domains cannot have CNAME records.
Question and response
Question
dig aa-rt.sharepoint.com. NS
Answer
aa-rt.sharepoint.com. 4 NS ns3-222.azure-dns.org.
aa-rt.sharepoint.com. 4 NS ns2-222.azure-dns.net.
aa-rt.sharepoint.com. 4 NS ns1-222.azure-dns.com.
aa-rt.sharepoint.com. 4 NS ns4-222.azure-dns.info.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0