I have never in my life received a W-2 that shows absolutely zero "Federal Income Tax" withheld in Box 2. Is this an error? What would be the causes of this?
That means no federal tax was withheld. Ask payroll.
Might happen if you had a significant amount of Combat pay....but yeah, you may have to check with payroll to see if it was a mistake.
Check your monthly LES statements too (you should always save every one for the year to spot such errors). SS and Medicare would automatically be taken out (Except for SS in the last 3 months of 2020), but if you don't see a "Federal Taxes" line with a $$ amount in the deductions column...then somehow you had Fed tax withholding stopped.