The 1099-R box 4 withholding should have been added to the 2018 1040 line 16 W-2 & 1099 withholding and also shown on the "Tax Payments Worksheet" line 12 - was it?
If not, then check your 1099-R entries for data input errors.
**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.**
