IF your paycheck stubs showed they withheld federal income taxes, then you need to contact that employer and get a corrected W2. IF they didn't withhold Federal income taxes, then you just enter the W-2 as it exists, into the software and see what happens after you've fully filled in all your data, income, deductions, credits...etc for everything else.
If your yearly income falls under a certain level, then no withholding may be proper as you wouldn't owe any taxes at all (in certain situations) ....but if you end up owing taxes this year, or every year....then you need to give the employer a new W-4 form to increase your Federal tax withholding.
If box 2 (Federal Income Tax withheld) is blank you should immediately contact your employer. There are two things that could have happened that would make your W-2 correct.
1.) Your earnings were so low that no tax was needed.
2.) You submitted a Form W-4 stating you were exempt from income tax.
Other than that there is no reason not to have income tax withheld, and your employer, or payroll department should be able to give you an answer.