In order for Chimera to perform minimization, atoms in standard residues have to have their PDB standard names so that Chimera can assign correct atom types and partial charges.  The error message you got typically results from one or more atoms not having their standard names.  Instead of going back and forth here on the mailing list, it would be best if after getting this error you used Help→Report A Bug to submit a bug report.  This will include the contents of Chimera's log, which will allow me to offer specific advice for how to correct this problem for your structure.