Entering 1042-S for a resident

Enter the amounts as if they were not on a 1042-S. If there is federal tax withholding you're going to have an issue.

Did you consider filing a dual status return instead of electing too be treated as a US resident for the whole year?

---

@Babak wrote:

My client was nonresident when he received 1042-S and then became a resident. I'm filing 1040 for him but the amounts on 1042-S don't show up anywhere.

---

Your client wouldn't have received a 1042-S because he's a nonresident alien when he received it.  It can only be because he was a nonresident when the income was paid to him or that he was a resident alien but did not file a W-9 with the payor.

Depending on whether your client was a dual-status resident in 2021, you'd need to discern when he received the FADP, which is subject to tax on a gross basis, and whether treaty rate(s) should apply.  FDAP received during the NR period should be reported on the 1040NR statement instead.  Unfortunately, this means you may not just enter that as if it's a regular 1099 - not unless your client was a full-year resident alien or elects to be treated as one.