After you file

if you are considered as resident for tax purposes for 2020, you will treat receiving the 1042-S as a resident in 2020.  You will need to report that income on your Form 1040.  Per instructions, income cod19 on the Form 1042s refers to compensation for teaching.  You can simply include it under Other Income. If you have filed your 2020 taxes, you would need to amend.  However, if tax treaty applies to you ( tax treaty can apply to both residents and non-residents) and exempt your income, you do not need to amend if the result does not create any differences.  As tax treaty topic is out of our scope for advising, you would need to consult a professional for verification. 

@fcinter1908

When did you enter the US and with what visa?

If you have not filed, you do need to include the income and show the exemption to the IRS it is not taxable after you verify with another professional.  Here are the steps:

In TurboTax online,

• Report your full income under the appropriate section. Then enter the treaty exempt amount as negative amount (e.g. -5000) under Federal Taxes / Less Common Income / Miscellaneous Income 1099A, 1099C / Other Reportable Income
• Download and fill in a Form 8833 from IRS to claim an exception.  https://www.irs.gov/pub/irs-pdf/f8833.pdf
• You cannot e-file.  You can still use the TurboTax program to prepare your tax forms, but you need to print them out and submit by mail along with the form 8833. 

@Lior