What year?  2019 line 12a is the tax on your income.  

2018 line 12a is for Child Tax credit/ or other dependent.  Is the $21 on the "a" line over on the left?  Or is it in the right amount column?   There is a 12b with a checkbox on the same line.  Is it checked X?

it is 2019 tax documents.  Would you know where turbotax came up with the number 21?

it looks like this 

12a  Tax(see inst.) Check if any from Form(s): 1 _ 8814     2 _ 4972    3 _            12a   21.

You can preview your return,

https://ttlc.intuit.com/community/accessing/help/how-do-i-preview-my-turbotax-online-return-before-f...  

The Tax comes from the Tax Tables or from one of the forms listed by line 12a.  It is the tax you owe on line 11b Taxable Income.  

$21 is the tax on $200-225 of taxable income line 11b.

Ohhhhhh ok this all makes so much sense now.  I was stressed about something being wrong but everything was correct!!  Thank you so much.  I was confused when i searched online what line 12a meant on a 1040 form because all of the results came up as Child tax credit, which i definitely dont have. You explained it so well. Thank you!!

also, would you know a lot about the Earned Income Credit?