Often it's because one has no income tax liability before applying this credit. This credit is a nonrefundable credit, so it can't be used to reduce your income tax liability below zero. It cannot be used to reduce other taxes such as self-employment tax or penalties on early distributions from retirement plans.
You're eligible for the credit if you're:
- Age 18 or older,
- Not claimed as a dependent on another person’s return, and
- Not a student.
You were a student if during any part of 5 calendar months of the tax year you:
- Were enrolled as a full-time student at a school, or
- Took a full-time, on-farm training course given by a school or a state, county, or local government agency.
A school includes technical, trade, and mechanical schools. It does not include on-the-job training courses, correspondence schools, or schools offering courses only through the Internet.
Here is theIRS page for the Retirement Savings Credit