I got a 1098 T form (2019), which shows $414 adjustment on box 4. I have applied American Opportunity credit for the same amount in 2018. Should I file amendment tax return for 2018, or I can make an

No, you won't amend 2018, but you might need to recapture the credit. 

Basically this means that you figure the amount it would have affected your 2018 return and then you claim that amount on your 2019 return. 

TurboTax can help you with this. 

According to the IRS:

"Refunds. A refund of qualified education expenses may reduce adjusted qualified education expenses for the tax year or require repayment (recapture) of a credit claimed in an earlier year."

TurboTax can help you do this:

Type   recapture education deduction   into the search box located at the top right of the screen

Click on the “Jump to recapture education deduction” link

Thank you so much it was very helpful.