The most likely reasons you owe additional tax is that you are a two-income family and your husband works a lot of overtime.
(Without knowing what other income, deductions or credits you have, it would be impossible to give you a better answer).
Even if claiming "0" withholding exemptions, you may still owe additional tax. The amount withheld from each of your paychecks is based on the earnings of that paycheck alone; when you combine your incomes (especially with overtime), you can be in a much higher tax bracket than either paycheck's withholding would cover.
To avoid owing additional tax at return time, consider having your employer withhold an additional amount from one or both paychecks. Figure out how much extra you owed, divide that by the number of remaining pay periods 2021 and have the employer withhold that much extra for the rest of 2021.
