You can't do anything now. Just prepare your income tax return. Hopefully, the amount you had withheld from your first W-2 will cover the taxes for your second W-2.
By the way, your employer did not make an error not withholding taxes from your second W-2. It just means that, based on what you made and what you authorized them to withheld (per your form W-4), no taxes needed to be withheld from that income.
TurboTax could help you complete a new W-4 form. See How do I update my 2020 withholdings and print a new W-4?
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