The message is pretty clear - it's telling you that there's no terminal (i.e. no pin) called biasn_lvgen in the opamp_a1p8 layout. Presumably you have an instance of this opamp in your design, and when the gen from source is done, it needs to assign the appropriate connectivity on each terminal of the resulting instance in the layout. The instance in the schematic will have connections to each terminal on the placed symbol; when the corresponding opamp layout is placed in your generated layout, it needs to know how to connect up each of those pins - if there's no pin on the opamp layout, it can't connect to it.
So something is wrong with the layout of your opamp_a1p8; it's missing a pin.