If you took the missed RMD in 2018, it will be reported on your 2018 tax return next year along with your 2018 RMD.
One of our TurboTax Superuser, Macuser_22 , had a great response to this question of how to request a waiver:
You need to take the missed RMD ASAP and file a 5329 with your mailed 2017 tax return.
It is almost impossible to prepare a 5329 form with TurboTax if you do not have a 1099-R to file. You are better off just downloading the forms from the IRS. Only if you have a 2017 1099-R to enter can the 5329 be created. If no 1099-R then also hand fill out the IRS 2017 form and attach it to your 2017 tax return that you will need to mail.
From 5329 instructions:Waiver of tax. The IRS can waive part or all of this tax if you can show that any shortfall in the amount of distributions was due to reasonable error and you are taking reasonable steps to remedy the shortfall. If you believe you qualify for this relief, attach a statement of explanation and file Form 5329 as follows.
1. Complete lines 52 and 53 as instructed.
2. Enter “RC” and the amount you want waived in parentheses on the dotted line next to line 54. Subtract this amount from the total shortfall you figured without regard to the waiver, and enter the result on line 54.
3. Complete line 55 as instructed. You must pay any tax due that is reported on line 55.
The IRS will review the information you provide and decide whether to grant your request for a waiver.
Here are links to the 2017 5329 form and instructions. See part IX of instructions, page 7.
https://www.irs.gov/pub/irs-pdf/f5329.pdf
https://www.irs.gov/pub/irs-pdf/i5329.pdf
