After digging into all logs, could found this is the error is with LetsEncrypt cert issuance. Anyone facing similar issue might help below.
first I saw this error in Nginx logs
cannot load certificate "/shared/ssl/forum.abc.com.au.cer": PEM_read_bio_X509_AUX() failed (SSL: error:0906D06C:PEM routines:PEM_read_bio:no start line:Expecting: TRUSTED CERTIFICATE)
then tried to check ssl logs
forum.abc.com.au:Verify error:CAA record for forum.abc.com.au prevents issuance
Before running discourse install script for sub domain we need to verify if main domain is having any CAA records and check the Certificate Authority if it’s not LetsEncrypt (In my case, main domain CAA is comodoca.com) , your letsencrypt certs for discourse won’t be issued
Fix: You need to add CAA record for discourse subdomain as mentioned here
Acme:error:caa :: CAA record for ... prevents issuance - Help - Let's Encrypt Community Support
Delete the old cert files and try rebuilding again
rm -rf /var/discourse/shared/standalone/ssl
rm -rf /var/discourse/shared/standalone/letsencrypt
./launcher rebuild app