Ask your boss/hr department

Could be you don't make enough to trigger income tax withholding, if SS and Medicare are being withheld that's probably the case

If NO taxes are being withheld then you are probably a contract employee responsible for your own taxes

Maybe you filled out your W4 wrong

Because you aren't making enough money for income tax to be taken from your checks.

There is no way for to know why based on the wording of your question.

Because that’s what you told your employer to do when you filled out your W-4. If that’s not correct, you’ll want to fill out another one.