Do I need to report additional federal tax that I paid in 2018 related to a 2016 return & which followed a letter that I received from the IRS saying that I had underpaid

No, you do not need to report federal tax you paid for prior tax years - there is no deduction or credit for it in the prior year.  If, on the other hand you paid state income taxes from a prior year, you might be able to deduct that amount if you itemize your deductions.