General::estep
The value of the ExponentStep option is an inexact number rather than an integer:
NumberForm[{1.7*^-23, 2.3*^18, 3.9*^35}, ExponentStep -> 5.0]This shows a valid value for the ExponentStep option:
NumberForm[{1.7*^-23, 2.3*^18, 3.9*^35}, ExponentStep -> 5]