If you have this error message “Command rejected: Not eligible for secure port.” it means that you first must set the port to the access mode.

SW4(config)#int f0/4
SW4(config-if)#switchport port-security
Command rejected: Not eligible for secure port.
SW4(config-if)#switchport mode access
SW4(config-if)#switchport port-security

and that’s it.