If your 2019 tax hasn’t processed yet, you need your 2018 informations. Please register an account for IRS.gov (view your account). After that, log in and go to “Tax Record” > “Get Transcript Online”> Choose “Federal Tax” > Go. Look for 2018 tax. Look for Adjusted Gross Income and Amount you owe to get the exact number which might be different from your 1040 (which happened to me). Now just go back to Get your payment site and try with the new informations. Good luck!
