zimbra SSL

Hi Guys,

Im trying to install Letsencrypt on Zimbra and couldnt find a proper article to do it. currently im getting following error, when following the procedure from this site

sudo su - zimbra -c '/opt/zimbra/bin/zmcertmgr deploycrt comm /opt/zimbra/ssl/letsencrypt/cert.pem /opt/zimbra/ssl/letsencrypt/zimbra_chain.pem'
** Verifying '/opt/zimbra/ssl/letsencrypt/cert.pem' against '/opt/zimbra/ssl/zimbra/commercial/commercial.key'
Certificate '/opt/zimbra/ssl/letsencrypt/cert.pem' and private key '/opt/zimbra/ssl/zimbra/commercial/commercial.key' match.
** Verifying '/opt/zimbra/ssl/letsencrypt/cert.pem' against '/opt/zimbra/ssl/letsencrypt/zimbra_chain.pem'
ERROR: Unable to validate certificate chain: C = US, O = Internet Security Research Group, CN = ISRG Root X1
error 2 at 2 depth lookup: unable to get issuer certificate
error /opt/zimbra/ssl/letsencrypt/cert.pem: verification failed

any idea how to fix this?

reference : https://computingforgeeks.com/secure-zimbra-mail-server-with-letsencrypt-ssl-certificate/