Apex domains cannot have CNAME records.
Question and response
Question
dig deutschepost.de. NS
Answer
deutschepost.de. 5 NS ns01.brandshelter.com.
deutschepost.de. 5 NS ppd00021.deutschepost.de.
deutschepost.de. 5 NS ns.deutschepost.de.
deutschepost.de. 5 NS ns02.brandshelter.net.
Authority
The response has no authority records.
Additional
ns.deutschepost.de. 5 A 149.239.160.100
ppd00021.deutschepost.de. 5 A 149.239.160.240
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0