In PAN-OS , if the duplex on a 1000 Mbps port is not set to "auto" the system will display the following error message upon committing:
Interface eth1/x link speed is set to '1000', link-duplex MUST be set to 'auto'
This error response is by design in PAN-OS to report a configuration error. The only valid duplex option on a 1000 Mbps port is auto, this is defined within the IEEE 802.3 specification for 1000base-T
To change the duplex option via CLI, run the following command:
set network interface ethernet ethernet1/5 link-speed auto