I am working on a decameric protein and want to create disulfide bridge
between cysteines of adjacent monomers.
...
KeyError: 'No such residue: 44:D'
Not sure how this message could be any more clear - you don't have a
residue 44 in chain D. Remember that Modeller always numbers from 1. Try
building a model without any bridges and look at the output file to see
the numbering.