You need to check your .gro file. You should see which atom is called LC3 and then should certify how this atom is called in your force field selected (see your .top for that)

You must be following Dr. Justin's tutorial. Please check carefully if you have properly added the lipid bilayer's parameters in ff directory. Also check if you have added #include statement:

Finally, change the #include statement in your topol.top from:

#include "gromos53a6.ff/forcefield.itp"

to:

#include "gromos53a6_lipid.ff/forcefield.itp"

Lastly, we need to include the specific parameters for our DPPC molecules. The process for doing so is quite simple. Just add the line #include "dppc.itp" in your topol.top, somewhere after the position restraints section for the protein, which defines the end of the protein [ moleculetype ] definition. Doing so is analogous to adding any other small molecule or solvent into the topology. In this section and throughout this tutorial, text in green will denote lines that you should add, while other text (in black) are lines that should already be present in the topology prior to the modification indicated.

; Include Position restraint file #ifdef POSRES #include "posre.itp" #endif

; Include DPPC chain topology #include "dppc.itp"

; Include water topology #include "gromos53a6_lipid.ff/spc.itp"

(From the tutorial)