Check that you have all 14 characters. There is no space or hyphen. From Income Tax Withholding - Utah Income Taxes - Utah.gov:
For payer ID: 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.
The program wants lower case for wth. Contact the employer or payer of the income to get the Utah ID number. If there is no state withholding, you can delete the entire state line.
1) Make sure you change it on the original form, and do not try to correct it in the error check.
I don't know if this is till the situation.,,,.BUT
2) One year, I found that...IF I had initially put it in wrong (CAPITAL letters), and went back later to correct them to lower case? The change didn't "take" and they changed back to capital letters as soon as I left the page.
...The solution was to blank out the entire Utah state withholding number, leaving the field empty. Then I "Continued" to the next page...then hit "Back" to go back to the proper page, and filled in the blank box with the correct number with the lower case wth. Moving on from that, the lower case remained.