Privileges are represented
by the numbers 0 through 15, with zero allowing only operator privileges
(and requiring two logins) and 15 representing root privileges. The
root privilege level is the only level that allows manager level access
on the switch.
Check the Privilege level
box and set the privilege level to 15 to allow "root" privileges.
This allows you to use the single login option.
As shown in Configuring the switch TACACS+ server access, login and enable access is always
available locally through a direct terminal connection to the switch
console port. However, for Telnet access, you can configure TACACS+
to deny access if a TACACS+ server goes down or otherwise becomes
unavailable to the switch.