I discovered that I did receive the $1400 Stimulus payment and I do not know where to enter it in TurboTax. Where should I do this?

It isn't taxable and doesn't need to be entered.  It just asks you if you received the funds.

TurboTax has a stimulus screen to enter your payment.

If you did not receive the full stimulus, the missing money will be added to your refund as a Recovery Rebate Credit.

If you don’t report the amount, or report the wrong amount, IRS will correct your tax return. This will delay your refund.

To get to the stimulus section in TurboTax online:

• Type "stimulus" in Search in the upper right
• Select "Jump to stimulus"
• TurboTax will show you the amount of stimulus you’re entitled to and ask whether the amount matches what you got
• If it does, say Yes. If it doesn't, say No
• If you say No, TurboTax will let you enter what you received and add the extra to your refund