If you contributed to a Traditional IRA for 2016 and it was not reported on your 2016 tax return, you must amend to report it. Either it would be a deductible contribution that reduces your tax liability and you would be due a refund, or it would be a non-deductible contribution that requires an *8606 form to be filed as part of the tax return to report the non-deductible basis in the IRA. Either way required an amended return.
* An 8606 can be filed by itself without amending if you choose not to deduct (or cannot deduct because of AGI limits).
2016 8606 form: https://www.irs.gov/pub/irs-pdf/f8606.pdf
Instructions: https://www.irs.gov/pub/irs-pdf/i8606.pdf
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
https://ttlc.intuit.com/replies/3288565
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