Port does not support this command OR
'undo loopback' is necessary for this command !
1. Loopback configuration for interface: execute display interface command to check if the interface is configured with loopback, as shown in the following red part:
[MA5200G-Ethernet1/0/4]disp inter e 1/0/4
Ethernet1/0/4 current state: up
Line protocol current state: up
Hardware is WAN-FastEthernet,
Address is 00e0-fc5f-bb14
The Maximum Transmit Unit is 1500 bytes, the BandWidth is 100000 Kbits
Send-frame-type is Ethernet_II, loopback not set
2. Otherwise, it indicates that FE optical interface does not support auto-negotiation, so it cannot be configured to that.