why i'm not getting approval for certificate management ?

Hello.

To perform domain verification, try registering the CNAME record displayed on that screen in the corresponding hosted zone.
https://docs.aws.amazon.com/acm/latest/userguide/dns-validation.html

By the way, regarding "harsha121.space", the NS record cannot resolve the name properly as shown below.
Please check whether the NS record is registered correctly.

dig harsha121.space ns

; <<>> DiG 9.16.1-Ubuntu <<>> harsha121.space ns
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 42963
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;harsha121.space.               IN      NS

;; Query time: 1511 msec
;; SERVER: 2405:6581:bd60:600:9a2d:baff:fe51:9c0#53(2405:6581:bd60:600:9a2d:baff:fe51:9c0)
;; WHEN: Fri Jun 07 17:32:59 JST 2024
;; MSG SIZE  rcvd: 44

If you manage your domain with Route53, you can open the hosted zone and check the NS records.
Please check if this record matches the NS record registered in the hosted zone.