Why does 1099-NEC Box 6 require a 14 character number when the number on my form is 9 characters?

This is because per the UT state website, valid withholding account numbers are 11 digits followed by WTH.  Since your 1099-NEC does not have the full account number, you need to contact the payer and have them issue you a corrected 1099-NEC.

Per UT website:

FYI: Withholding Account Number

The Utah withholding account number is a 14-character number. The first eleven characters are numeric and the last three are “WTH.” Do not enter hyphens. Example: 12345678901WTH.

If form W-2 or 1099 does not include this number, contact the employer or payer to get the correct number to enter on TC-40W, Part 1. Failure to include this number on TC-40W may cause your withholding to be disallowed and delay any refund.