Fatal Error: atom X in multiple T-coupling groups

GROMACS version: 2020.6
GROMACS modification: Yes/No
I am getting this following error although I have added both the Protein/ Non protein groups
Fatal error:
Atom 4812 in multiple T-Coupling groups (1 and 2)
Kindly help me

What is atom 4812? What have you specified in your tc-grps setting? Somehow you have created two groups that overlap such that they both include this atom, which is not allowed.

I created an index file wherein I merged my protein and ligand molecules.
This is what I have specified in my tc groups

; Temperature coupling
tcoupl = V-rescale ; modified Berendsen thermostat
tc-grps = Protein_UNL non-Protein ; two coupling groups - more accurate
tau_t = 0.1 0.1 ; time constant, in ps
ref_t = 300 300 ; reference temperature, one for each group, in K

A ligand molecule falls within the default Non-Protein group. You need to create a different group that does not include the ligand if you are merging it with the Protein group.