>CPA says I owe federal taxes; payroll says they withheld everything correctly

It's possible for both of these things to be true.

Your employer only knows how much they are paying you. They don't know about the rest of 900 possible things that could change your tax bill such as capital gains, interest, rental income, etc.

ETA: I see you've added that you have no other sources of income. You can look at the IRS tax withholding tables yourself and compare what your employer is deducting to the tax tables. No need to pay an accountant to do that.