To calculate my 2020 AGI, do I subtract the standard deduction of $12400 from my box 1 w2 wages. I do not have any other deductions

No, your AGI comes before you deduct your Standard Deduction.

Your AGI minus your Standard Deduction is your Taxable Income, not your AGI.

Did you have any interest from your bank or credit union? That must be added to your Box 1 wages to get your AGI. To get the full picture of what to add and subtract from Box wages to get to your AGI, download a form 1040. If you had ONLY Box 1 wages as your income (no interest, no unemployment, no stock sale proceeds, etc.) and no deductions (charitable contributions, etc.) on your page 1 of the form 1040, then your wages in Box 1 are equal to your AGI. Highly recommend you download a copy of the Form 1040 and confirm nothing on lines 2b-10c applied to you in 2020.

You would find your AGI on your 2020 tax return. Please see How do I find last year's AGI?

Was your return rejected due to an incorrect AGI? If so, please see How do I fix e-file reject IND-031-04 or IND-032-04?

I assumed the question was posted because the person did not have a copy of their 2020 tax return so they were trying to back in to their 2020 AGI. Looking at a 2020 tax return is definitely the easiest solution.