symbol list and nomenclature
Is it necessary add a nomenclature with a list of used symbols of
  variables, operators and abbreviations?


You should treat this comment like any other comment from a reviewer: It tells you that one potential reader of your paper (and, if the editor did a good job, a qualified one) had a hard time understanding your paper. Hence, you should improve the presentation of your paper in this respect, so you should make the effort and help potential readers. (There may be the chance that this particular journel even requests a nomenclature, but I did not check this…)


  Is there any specific position for this section ?


For this, there are two good answers here.