Apex domains cannot have CNAME records.
Question and response
Question
dig _acme-challenge.sandbox.esigno.com. NS
Answer
_acme-challenge.sandbox.esigno.com. 4 NS diana.designplus.cz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0