Apex domains cannot have CNAME records.
Question and response
Question
dig heartintraining.com. NS
Answer
heartintraining.com. 5 NS ns-1377.awsdns-44.org.
heartintraining.com. 5 NS ns-1779.awsdns-30.co.uk.
heartintraining.com. 5 NS ns-460.awsdns-57.com.
heartintraining.com. 5 NS ns-1392.awsdns-46.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0