Positional argument -f cannot be accepted

GROMACS version: 2019.1
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Command: gmx cluster –f trajout3.xtc –s em3.tpr –cutoff 0.35 –cl cluster3.pdb –method gromos –g cluster3.log

I got this error: Positional argument ‘–f’ cannot be accepted. Perhaps you forgot to put a
hyphen before an option name.

What could be wrong here?

The error actually tells you, you did not use a hyphen before the options.

You used where as you are meant to use -, hyphen or the negative sign.