I have tried to run your script with 2016.4 and I cannot reproduce the problem. I can add the relaxed configuration to the Builder without errors. Please share the specific error message and preferably also a screenshot.
However, looking at the log-file, the relaxation has not converged at all, and the initial forces are extremely large, on the order of 10^6. To me, it looks like either your potential is not very good for this system, or the initial geometry is not appropriate.