The delegation for track is in place and the selzy.com nameservers are responding with NS records for it so seems to be ok.
jokerbet.biz. 3600 IN NS emily.ns.cloudflare.com.
jokerbet.biz. 3600 IN NS trey.ns.cloudflare.com.
;; Received 131 bytes from 156.154.127.65#53(c.gtld.biz) in 3 ms
track.jokerbet.biz. 3600 IN NS uns1.selzy.com.
track.jokerbet.biz. 3600 IN NS uns2.selzy.com.
track.jokerbet.biz. 3600 IN NS uns3.selzy.com.
;; Received 113 bytes from 2606:4700:58::adf5:3bf2#53(trey.ns.cloudflare.com) in 3 ms
track.jokerbet.biz. 3600 IN NS uns2.selzy.com.
track.jokerbet.biz. 3600 IN NS uns3.selzy.com.
track.jokerbet.biz. 3600 IN NS uns1.selzy.com.
;; Received 113 bytes from 195.13.229.163#53(uns2.selzy.com) in 39 ms
You are asking for the NS records the same way as if you asked for any other record (A, AAAA, MX, TXT, etc). You have delegated send to selzy.com nameservers so even requests for NS records will be delegated to them, and as you haven’t set the records there, there’s no answer.
Same as when you ask for the NS records for track, the answer is coming from the selzy.com nameservers, which you have set up.
The NS records you have set at Cloudflare can be seen when you do a trace as I did above and indicate the delegation. Your Cloudflare configuration is correct, you need to add the records for send in the selzy.com nameservers.