If one's AGI (Adjusted Gross Income) is ZERO, this is because one has placed the once-taxable income in brackets and subtracted it, making a demand. If one has been making demand for lawful money throughout the year and has some checks that support that, add it to Schedule 1. This will support that there is an amount of withholdings, which supports filing 'in the first place'. Since one has earned no taxable income, one might want to show that reporting to the IRS for any withholdings.
Consider whether one is in California, New Jersey or others that operate their own IRS Code via a Franchise Tax Board. These boards will start up with Frivolous Penalty paper and an IRS agent will see this as an opportunity - that is, if the "taxpayer" does not know proper Refusal for Cause (R4C) techniques, including proper record forming of lawful money redemption. There are always solutions so do not become discouraged. Be competent in understanding the common law to realize Congress allowed for the lawful money remedy. Meaning once one starts requesting a sit-down audit, the law will prevail and things will be resolved.
NOTE: If one has used TurboTax or similar filing software, the state effectively drops income to ZERO and the state may send a statement showing they "corrected" the state return to ZERO, instead of getting a full refund. Sometimes the state is clever by pretending one has zero income rather than looking at the state withholdings that they have on account, and thus owe. Assess how much the state is keeping versus how much trouble the state may bring to your mailbox if you try correcting it. A refund can be obtained, but sometimes it takes a lot of energy. Eventually when the "judge" sees what they are up against, they will not stand in the way of one's redemption.
NOTE II: We find that using "Social Security Secrets" is the best way to opt-out of the withholding/SSI system, rather than trying to fight a battle "within the system".