If 2018 is your first year for which RMD was required, the deadline for taking
2018 RMD is April 1, 2019. As it is not late yet, no special reporting is
required. You'll receive 1099-R next year and report it on 2019 tax return. Just please be sure to take the RMD for 2019
If it was withdrawn in 2019 it will be reported on your 2019 Tax Return. However, you may need to file Form 5329 with your 2018 Tax Return to request a waiver of the penalty. The waiver would only be denied if there is no reasonable explanation and the missed RMD was not taken at all.)
If you do have a 2018 1099-R to report then the RMD question in the interview will go to the form 5329 waiver interview if you say that the 2018 RMD was not taken.
If using the Online version and have no 2018 1099-R to file, you will have to prepare the 5329 manually.
Download the 5329 form from the IRS website and fill it out the same as above. Print and mail your return with the 5329 form and explanation attached as described in the 5329 instructions.
The 2018 5329 must be attached to your printed and mailed 2018 tax return and cannot be e-filed.
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.