Even if you had only one job last year, income from other sources (interest, dividends, investments/distributions) can increase your income and cause a tax due.
@johncp1989
The "S" you see in for the Federal taxes withheld, without a "1" or a "0" the is probably correct. There would/should be no "0" on it anymore since the IRS changed their W-4 procedures to not have any numbered "allowances" anymore.
State withholding forms (equivalent to the old W-4), may still have numbered exemptions/allowances (however they decide to label them), since they can choose on their own how the want to create/prepare/calculate using those state-specific forms.
______________________
Your statement about supposedly having no ("0") Federal allowances might have been the case if you are referring to an older W-4 form, but the employer was likely required to convert it to the new W-4 withholding format and tables that the IRS now uses for 2021. As @Critter-3 suggested, you should try submitting a fresh copy of the newly-formatted W-4 to your employer, and perhaps have an additional amount subtracted from each pay to approx match the $400 you were short, by the end of next year.
___________________________________
Here's a copy of the new W-4 form
(line 4c would be where you would indicate an added amount you want withheld for each paycheck period, not for the entire year. You have to make that calc yourself based on how often you get paid)
About Form W-4, Employee's Withholding Certificate | Internal Revenue Service (irs.gov)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
