for 2020 if your w-2 shows no withholding and that's consistent with your paystubs there is noting that can be done.  for 2021, talk to your employer.  you may have made an error in completing form w-4 which determines your wirhholding.

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?