Fatal errors

GROMACS version: 2020.1-Ubuntu-2020.1-1
GROMACS modification: Yes/No
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Fatal error:
The residues in the chain GLY1–UNK37 do not have a consistent type. The first
residue has type ‘Protein’, while residue UNK18 is of type ‘Other’. Either
there is a mistake in your chain, or it includes nonstandard residue names
that have not yet been added to the residuetypes.dat file in the GROMACS
library directory. If there are other molecules such as ligands, they should
not have the same chain ID as the adjacent protein chain since it’s a separate

Ive connected the CYS with Avogadro2 and Im facing this problem. How can I sole it?