Apex domains cannot have CNAME records.
Question and response
Question
dig ipv4.knet.es. NS
Answer
ipv4.knet.es. 3 NS ns-cloud-b1.googledomains.com.
ipv4.knet.es. 3 NS ns-cloud-b3.googledomains.com.
ipv4.knet.es. 3 NS ns-cloud-b2.googledomains.com.
ipv4.knet.es. 3 NS ns-cloud-b4.googledomains.com.
Authority
The response has no authority records.
Additional
ns-cloud-b1.googledomains.com. 2 A 216.239.32.107
ns-cloud-b2.googledomains.com. 2 A 216.239.34.107
ns-cloud-b3.googledomains.com. 2 A 216.239.36.107
ns-cloud-b4.googledomains.com. 2 A 216.239.38.107
ns-cloud-b1.googledomains.com. 2 AAAA 2001:4860:4802:32:0:0:0:6b
ns-cloud-b2.googledomains.com. 2 AAAA 2001:4860:4802:34:0:0:0:6b
ns-cloud-b3.googledomains.com. 2 AAAA 2001:4860:4802:36:0:0:0:6b
ns-cloud-b4.googledomains.com. 2 AAAA 2001:4860:4802:38:0:0:0:6b
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0