You should always a) give your ZCS version (as I've mentioned before), b) tell us which instructions you've followed and c) did you add the DKIM records to a LAN DNS server or a DNS server on the internet.
@phoniux sorry for that but version is same last time it got on my nerves , so I did reset my vps and fresh os install and fresh zimbra installation on it with latest version
my zimbra version : Release 8.8.15.GA.4179.UBUNTU20.64 UBUNTU20_64 FOSS edition, Patch 8.8.15_P33.
this is a fresh setup I dont get it if mxtools box validating dkim key then why is it failed on zimbra.
issue is fixed . I have checked /etc/resolv.conf and found nameserver 127.0.0.1 , I changed that localhost with 8.8.8.8 . I've installed dnscache as well so I did add 8.8.8.8 into zimbra .