Error While Simulating Carbon Nanotube in Water

GROMACS version:
GROMACS modification: Yes/No
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I was trying to simulate carbon nanotube using the tutorial Carbon Nanotube - Gromacs
However, when I tried running the command

gmx x2top -f cnt.gro -o -ff cnt_oplsaa -name CNT -noparam

It showed me this error-
Could not find force field ‘cnt_oplsaa’ in current directory, install tree or
GMXLIB path.

I did check again and I do have a directory called cnt_oplsaa.ff and it has three files cnt.itp, cnt.n2t and cnt.rtp.

I also tried using cnt_oplsaa.ff in the command instead of cnt_oplsaa but I still got the same error. Can someone please assist me with this?

Thank you,

Force fields are recognized from the file forcefield.itp in a directory with the extension .ff. Maybe that is the issue?
Best regards