Hi Mahmut, in response to your comment on not being able to locate where to change permissions, this is not in the REPORTER where you're witnessing the "reporter session task failed'
Thank you so much, But I couldnt find user account permissions menu in veeam one monitor or reporter.
I dont know how to change permissions or add another account.
The first place to check is simply to ensure the details you entered for your vCenter are correct (could be needing FQDN or maybe the permissions of the account you've used are not suffice). These are best configured in the VEEAM ONE MONITOR
(not reporter), so open the MONITOR, go to INFRASTRUCTURE VIEW, find your VC from the left hand navigation pane, right click, select "CONNECTION SETTINGS" and re-enter your details (probably a good place to start).(see below)
Failing this, firewall and general network connectivity between the Veeam ONE estate and your VC.