Box 6 was left empty on my 1099-NEC but TurboTAx won't let me file without anything on it

If there was no state tax withheld in Box 5, then please leave Boxes 5, 6 and 7 empty.

If there was state tax withheld in Box 5 and box 6 is empty, you can use the identification number as shown in the Payer's ITIN number Box under the Payer's name.