Apex domains cannot have CNAME records.
Question and response
Question
dig do.pc2linux.com. NS
Answer
do.pc2linux.com. 5 NS tektonic.pc2linux.com.
do.pc2linux.com. 5 NS digitalocean.pc2linux.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0