Apex domains cannot have CNAME records.
Question and response
Question
dig bb.euroweb.sk. NS
Answer
bb.euroweb.sk. 5 NS auth2.ns.swan.sk.
bb.euroweb.sk. 5 NS auth1.ns.swan.sk.
bb.euroweb.sk. 5 NS auth4.ns.swan.sk.
bb.euroweb.sk. 5 NS auth3.ns.swan.sk.
Authority
The response has no authority records.
Additional
auth1.ns.swan.sk. 5 A 217.75.71.143
auth2.ns.swan.sk. 5 A 193.86.13.72
auth3.ns.swan.sk. 5 A 3.67.61.110
auth4.ns.swan.sk. 5 A 217.75.71.144
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0