I am working on the tax year 2020 and how can I enter the check amount which I already mailed to IRS for year 2020 in turbotax? Thanks

Depends on whether you paid an estimated tax or you paid taxes with an extension request.

 

To enter, edit or delete estimated taxes paid (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Estimates, click on the start or update button

Or enter estimates paid in the Search box located in the upper right of the online program screen. Click on Jump to estimates paid

 

To enter, change or delete a payment made with an extension request (Federal, State, Local) -

• Click on Federal Taxes (Personal using Home and Business)
• Click on Deductions and Credits
• Click on I'll choose what I work on (if shown)
• Scroll down to Estimates and Other Taxes Paid
• On Income Taxes Paid, click on the start or update button

On the next screen select the type of extension payment made and click on the start or update button

Or enter federal extension payment in the Search box located in the upper right of the program screen. Click on Jump to federal extension payment

The federal extension payment will be shown on Schedule 3 Line 9. The amount from Schedule 3 Part II Line 13 flows to Form 1040 Line 31

IF you marked your check "Form 1040 year 2020", you don't put that on your tax return, unless you paid it with form 4868. It is just your regular payment of the tax you owe.