How to replace a dataframe value with NA based on location?

Question

This is a duplicate of the following question, as I'm trying to adapt it for my data...

[https://stackoverflow.com/questions/66236799/how-to-replace-a-value-with-na-based-on-two-conditions-in-r-dplyr][1]

Problem: I'm plotting a series of mean and standard deviation data but the lower errorbar at one site, exceeds the x-axis...

Aim: I want to repeat the standard deviation column but with the high SD value replaced with NA - then call on this new column to plot the lower errorbar vector (so it effectively only plots the upper errorbar for this site). Alternatively, if there is an easier way to only plot an upper errobar for the specific site, without modifying the data frame... great!?

Simplified data frame:

df$Site = c(1:3)

df$SD = c(0.74638, 1.82636, 0.63874)

I've tried the following code but it just reprints the same values in the new column:

df1.1 <- df %>% mutate(SD_s4 = ifelse(Site == "2" & SD == 1.82636, NA_real_, SD))