The reason you see this error message is because in a 2950 the port first needs to be set to an access port and then only port-security can be turned on.
Switch(config)#int fa0/1
Switch(config-if)#switchport port-security
Command rejected: Not eligible for secure port.
Solution:
Switch(config-if)#switchport mode access
Switch(config-if)#switchport port-security