Retirement tax questions

Incorrect.  Allowable prior-year 401K contributions must be deducted from total gross income regardless of current year contributions.  But Turbotax "Premier" 2018 precludes that appropriate treatment by lumping together prior- and current-year  401K contributions and erroneously generating incorrect tax liabilities.  Read the 2018 W2 Box 12 instructions very carefully.  Suggestions?