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