The system is saying that my UT Withholding ID is incorrect, but I have triple checked it and I have put it in correctly. am I missing something?

It depends. According to the Utah State Tax Commission, the state ID should be 14 characters, the first 11 are a numbers, and the last three are "WTH". ‌Have you entered an ID that looks like this: 12345678901WTH?  If the numbers you have are a string of less than 11 digits, some customers have had success adding zeros to the ID you have.‌ Otherwise, you should contact your payroll department for the correct state ID. 

Did you really try the one where they use wth all in lower case? 

That's worked in the past for most people, but it must be done on any form that has UT withholding.

Sometimes you have to delete everything in the box, click in another box, then come back to the box and enter it all again, being careful to make the letters lower case

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  (apparently there is another form-type with UT withholding, that doesn't use "wth" ,but uses three other letters...whatever letters are on your paper form, use lower case.