Why when I amend to itemized deductions from standard deduction in my 2018, it has tax due? I already paid my tax due on my 2018 and I had the standard deduction.

Why are you amending?  The tax laws changed for 2018 and beyond.  If you are trying to force using itemized deductions instead of your standard deduction, you might not be better off with itemized deductions.  Your standard deduction nearly doubled for 2018--so it has become difficult for many people to have enough itemized deductions to exceed their standard deduction.  The software chooses the one that is better for you.  So maybe you should forget about itemizing and leave well enough alone.

STANDARD DEDUCTION

Many taxpayers are surprised because their itemized deductions are not having the same effect as they did on past tax returns.  The new higher standard deduction and the elimination of certain deductions, as well as the cap on state and local taxes have had a major impact since the new tax laws went into effect beginning with 2018 returns.

Your itemized deductions have to be more than your standard deduction before you will see a change in your tax owed or tax refund.  The deductions you enter do not necessarily count “dollar for dollar;” many of them are subject to meeting  tough thresholds—medical expenses, for example, must meet a threshold that is pretty hard to reach.  The software program uses all the IRS rules that apply to the expenses you enter, and it tells you if you have enough to use your itemized deductions or if using the standard deduction is more advantageous for you.  Under the new tax laws, some deductions have been capped—there is a $10,000 limit to the itemized deductions for state, local, property and sales taxes.

2018 Standard Deductions:

Single   $12,000  (+ $1600 65 or older)

Married Filing Separately    $12,000  (+ $1300 65 or older)

Married Filing Jointly  $24,000  (+ $1300 each spouse 65 or older)

Head of Household  $18,000  (+ $1600 65 or older)

Look at line 8 of your Form 1040 to see your standard or itemized deductions