I have a UC 1099G to add to my 2021 return. Which box does the amount withheld go in, the "federal tax withheld" box or the "state tax withheld" box?

On a standard Form 1099-G the amount in box 4 is federal income tax withheld, and the amount in box 11 is state income tax withheld. But some states do not use the standard form. The description of the amount of tax withheld on the 1099-G should identify whether it's federal tax or state tax. If not, you might have to ask the unemployment office or other department of your state government that issued the 1099-G.

Some states do not tax unemployment benefits. If your state does not tax the benefits, then any tax that was withheld has to be federal tax.

It should say on the 1099G.  Box 4 is for federal and box 11 is state withholding.   If you don't have the actual 1099G form it should say the box # by the amount.   If you can't figure it out you need to ask the state unemployment office.  

Here is a blank 1099G form

https://www.irs.gov/pub/irs-pdf/f1099g.pdf