If your (sub)domain was created by Client Area, make sure that (sub)domain exist on Addon Domain menu of cPanel.
Or if you created the subdomain by yourself, make sure it exist on Subdomain list.
If still error:
If your (sub)domain was created by Client Area, make sure that (sub)domain exist on Addon Domain menu of cPanel.
Or if you created the subdomain by yourself, make sure it exist on Subdomain list.
If still error: