You need to define the layer memristor, before you are using it in your rule file.
The way you can define this layer is something like below. x and y should be GDS number and purpose. The layer number you are mapping should be unique (i.e. it should be different from all other layer map numbers.)
LAYER memristor <random unique number>
LAYER MAP x DATATYPE == y <random unique number>
Got it. Thanks Venkatesh.