Why i have to pay $20 underpayment penalty for this year?

[Roy908]

1.  You may not have adjusted your withholding amounts from your wages, etc. to account for your increased income from the tax law changes.  This applies to your quarterly payments amounts also if you are making quarterly payments.
2.  I had a penalty for 2018 that was caused by an unexpected increase of Capital Gains in the fourth quarter.  Evidently some of my investments sold securities to take advantage of their higher prices at year end.  My quarterly payments were not sufficient to cover this increase in taxable income.  After looking at the trouble of annualizing this income I decided to accept the calculated penalty instead.  It all depends on the trade off of the amount of the penalty versus the detailed effort of annualizing the income.