This means you did not check the box for “Federal Income Tax” under “Exemptions” when you set this deduction. Here’s a sample setup:
For both Title and Abbreviation enter “SIMPLE IRA”.
Under Calculation select Variable (most common) , but you can change to match your needs.
Make it paid by the “Employee”.
Check the Box next to “Use on Box 12” and enter S for the W-2 Code.
Under Exemptions check the box next to “Federal Income Tax” (most common) and any other tax you might want to make the SIMPLE IRA deduction exempt from.