This error indicates that you have attempted to retrieve a non-existent setting.
Things to try:
show command in an iPXE script to display the value of a setting, consider using the echo command instead. For example: #!ipxe
dhcp
echo IP address: ${net0/ip}
You may also want to use the isset command. For example:
#!ipxe
dhcp
isset ${net0/ip} && echo IP address: ${net0/ip} || echo No IP address