If you were using a formula to set the values, then you code would probably work. However, since you are using Set Each Value, I assume that you need to specify the specific row value to do the calculation on. The below script worked for me
dt << New Column( RamanPeakList[NameIndex] || " Peak",
<<Set Each Value(
If(
Column( RamanPeakList[NameIndex] || " Peak Intensity" )[Row()] == Column(
"Temp" || RamanPeakList[NameIndex]
)[Row()],
:Raman Shift
)
)
);
Jim