A common mistake is to enable remote access on the router for the TTY and VTY lines, require that a password be given, and then not set a password. I have often installed a router in a network and then tried to access it via telnet and been denied access because a password is required but not set.
The correct configuration mode
It is a common mistake for users to attempt to enter sublevel configuration commands, such as interface configuration commands, right after entering enable mode. Be sure that you are in the correct configuration mode before entering commands in the IOS.
Source: Cisco Router Configuration & Troubleshooting by Mark Tripod, New Riders, 1999
To find out more about Cisco Router Configuration & Troubleshooting, go to http://www2.newriders.com/cfm/prod_book.cfm?RecordID=44 and https://www.informit.com/product/0735700249/.