Powerconnect M6220 Switch Unable to set port Speed

I am trying to set a port speed to 100 instead of the default 1000 using the following command:

1 Enter Interface Configuration mode for port 1.

console#configure
console(config)#interface gigabitEthernet 1/0/1
2 Change the speed and duplex settings for the port.
console(config-if-Gi1/0/1)#speed 100
console(config-if-Gi1/0/1)#duplex full
console(config-if-Gi1/0/1)#exit

But I got an error: "An invalid interface has been used for this function." Please refer to 

Trying to update the port speed on the GUI also returned an error:

Could any one advise what I am doing wrong? Thank you!!