Pdb2gmx command stops/hangs

GROMACS version: 2021.1
GROMACS modification: No

Hi, I am very a much new user. Seemed to have installed gromacs ("$ gmx pdb2gmx –version" gives the correct version). I have installed it on a Ubuntu 20.4 PC. I am trying the first tutorial at NMR Spectroscopy Research Group

When I run the following command, it stops/hangs:
$ gmx pdb2gmx -f protein.pdb -o protein.gro -p protein.top -ignh
It hangs at this step: “15: OPLS-AA/L all-atom force field (2001 aminoacid dihedrals)”

Any assistance will be much appreciated?
Best wishes
Gert Kruger

gmx pdb2gmx lists all the force fields implemented in GROMACS, did you try to select one by typing the corresponding number?
Best regards

Thanks! I did not read properly! Yes, I was supposed to choose “GROMOS 53a6 force field and the SPC water model”. That works.
Many thanks!