I setup Magento from a browser but after half installation, it stops then I again tried and it starts showing me above error. Then I figure out that maintenance flag is set.
In Magento 1 we have maintenance.flag file directly on root but In magento2 this is found in var folder by the name .maintanence.flag. So to fix above issue just remove the file var/.maintanence.flag.
MAGENTO maintenance mode
Magento uses maintenance mode to disable bootstrapping, it is useful when an application is going through maintenance, up gradation, or reconfiguration your site.
How will Magento knows id application is in maintenance mode:
var/.maintenance.flagexist, maintenance mode is on Unless some IP are exempt in
var/.maintenance.ipcontains the list of IP addresses. Thus if anyone accessing from using HTTP and the client IP address that corresponds to one of the entries in that list, then maintenance mode is off for only those listed IPs.