I can contribute to an IRA but when I indicate that I am making the the contribution between Jan. 1 and Apr 18 the tax deduction is negated. Why?

Turbo Tax suggested the contribution and on the worksheet, I could see the tax deduction.  So I indicated later that I was making the contribution (2,500) but when I then showed that I was making the contribution in 2017, the tax deduction was negated.