When trying to edit DHCP options of a DHCP reservation, you get this error:
Only reserved client options can be configured here.
To configure server or scope options, select the Server or Scope Options node and choose Configure Options.
You get this error because the interface is badly designed. Instead of taking you to the option (that Microsoft obviously know you want, because they’ve made an error message telling you about it), you get this error.
To configure client options for the DHCP reservation, right click the DHCP reservation you want to configure the option for and click Configure Options. You can then select the particular option from the list and edit it.