Selected server Veeam-bkup(os username) is unavailable

Hi @shahbaz ahmad this could be a few possible reasons try to remove and re-add the again repo.

check mount server connectivity towards repo check credentials

Check if all relevant Veeam services are running on the Veeam server (Veeam-bkup). Restarting the Veeam services can sometimes resolve connectivity issues.

do this and keep us posted.

Try editing the DD Boost repository and going through the wizard.  If that is successful then try the rescan again.  It could be a credentials problem, DNS/IP, etc. hard to say right now but this would be my first step to troubleshoot this.