I am trying to refine my model with loop refine protocol. But it is
always said:
...
KeyError: 'No such residue: 206:'
Just like it says, one of the residues you are trying to select does not
exist. Note that Modeller numbers residues starting from 1 in the model
(not using the template numbering). If you have more than one chain in
your model, you will also need to identify which chain you want to model
(e.g. "206:A").