I think I got the solution.
Add
#include "ligand.itp"
under
#include "amber14sb.ff/forcefield.itp"
Add
#include "ligand.top"
before
#include "amber14sb.ff/tip3p.itp"
Add
ligand 26
to the last line
Then, in the ligand.top, disable the sections of [defaults], #include "ligand.itp", [molecules]