Hi , @cgeorgee
According to your screenshot, your list name is not the same in your side. You need to check your list Name.

For example , if your list name is "Nonconformance Record NCR".
You need to use this in Uri:
_api/web/lists/getbytitle('Nonconformance Record NCR')/items(1)
And in your Body:
{
"__metadata": {"type":"SP.Data.Nonconformance_x0020_Record_x0020_NCRListItem"},
"AssignedToId":"-1"
}

If this reply can help you , you can click mark this reply as solution (Accept solution) which can help more people, thanks in advance!
Best Regards,
Yueyun Zhang