I'm looking at my "FITWH" and the filing status just says "S" and below it I see my state filing status that says "S" Exemptions "0" my federal taxes withheld just has an S next to filing status. there is no zero.
( I ONLY HAD ONE JOB LAST YEAR )
This can happen if you have more than one job during the year. Each job is withholding like it is the only job you have all year. To correct this fill in a new W-4 to either reflect the other income or have an extra amount withheld on line 4c.
Of course review the entire return for errors before you file.
It can also happen if you don’t have enough tax taken from paycheck. Go to your HR department and fill out a new W4 to increase withholding from paycheck to prevent a balance due in the future.
"Exempt" means you chose not to have any taxes withheld. It does not mean that you are exempt from paying taxes at all.
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.
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.
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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.
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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)
You can also use our W-4 Calculator 2022 to calculate the right amount of withholding for future tax years.