I did work full time and do an online course that was considered full time. Why can I not claim both? What do I do?

I was about to hand in my return when one of the warnings said I could not claim full time school and full time working income. I did work full time and I did do an online course that was considered 9 months of full time school. Why can I not claim both? What do I do?

Filing

The warning is likely stating you're not eligible for the Working Income Tax Benefit. With a few exceptions, that benefit is not available to students.