Apex domains cannot have CNAME records.
Question and response
Question
dig heartvalveeducation.com. NS
Answer
heartvalveeducation.com. 5 NS ns-993.awsdns-60.net.
heartvalveeducation.com. 5 NS ns-648.awsdns-17.net.
heartvalveeducation.com. 5 NS ns-364.awsdns-45.com.
heartvalveeducation.com. 5 NS ns-1520.awsdns-62.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0