By default it's a routed port. Make it a switchport by typing "switchport". Here is an example
LS-6506-4A(config)#int fastEthernet 5/1
LS-6506-4A(config-if)#switchport access vlan 10
Command rejected: Fa5/1 not a switching port.
LS-6506-4A(config-if)#switchport
LS-6506-4A(config-if)#switchport access vlan 10
LS-6506-4A(config-if)#