You have delegated demo.bytekol.com to the nameserver ns.bytekol.com with the NS record for demo, and ns.bytekol.com doesn’t resolve it.
If you want the A record for demo in your Cloudflare account to be used, then you’ll need to remove the NS record for demo.
dig +trace +nodnssec demo.bytekol.com
; <<>> DiG 9.10.6 <<>> +trace +nodnssec demo.bytekol.com
;; global options: +cmd
. 517445 IN NS a.root-servers.net.
....
. 517445 IN NS m.root-servers.net.
;; Received 239 bytes from 127.0.2.2#53(127.0.2.2) in 20 ms
com. 172800 IN NS i.gtld-servers.net.
....
com. 172800 IN NS m.gtld-servers.net.
;; Received 841 bytes from 192.5.5.241#53(f.root-servers.net) in 14 ms
bytekol.com. 172800 IN NS bingo.ns.cloudflare.com.
bytekol.com. 172800 IN NS bradley.ns.cloudflare.com.
;; Received 365 bytes from 2001:503:d2d::30#53(k.gtld-servers.net) in 176 ms
demo.bytekol.com. 300 IN NS ns.bytekol.com.
;; Received 78 bytes from 108.162.194.36#53(bingo.ns.cloudflare.com) in 24 ms
demo.bytekol.com. 604800 IN SOA ns.demo.bytekol.com. email.demo.bytekol.com. 2 604800 86400 2419200 604800
;; Received 90 bytes from 18.61.216.83#53(ns.bytekol.com) in 202 ms