Periodic to non periodic (two material layers - tensile test)

Your epoxy layer is likely periodic across the XY plane so you cannot simply change the boundary condition from ppp to pps without breaking any bonds. That is why the error “ERROR on proc 0: Bond atoms 582 585 missing on proc 0 at step 0” occurred.

You need to either rebuild your interface with a proper termination so it is not periodic along Z or you need to use other LAMMPS commands to do tensile deformation.