Dear Subhasree,
This is a known bug of that release of the GROMACS port of the CHARMM36 force field, occurring if COO- terminus is selected and the last residue is a glycine. It has been reported here: Newest CHARMM36 port for GROMACS - #7 by jalemkul. Until the next release of the port, where this will be fixed, you can use the workaround suggested there.
Kind regards and sorry for the inconvenience,
Andras