If you did not deduct the 2018 contribution then in order to claim it as non deductible you need to file a 2018 8606 showing it as non-deductible. That would also require that you file amended 8606's for any following years that you filed 8606 form for since the line 2 & 14 carry forward amounts will be different.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**