Getting "NA" when I run a standard deviation

You probably have missing values in var, or the column is not numeric, or there's only one row.

Try removing missing values which will help for the first case:

sd(dat$var, na.rm = TRUE)

If that doesn't work, check that

class(dat$var)

is "numeric" (the second case) and that

nrow(dat)

is greater than 1 (the third case).

Finally, data is a function in R so best to use a different name, which I've done here.

I've made the mistake a time or two of reusing variable names in dplyr strings which has caused issues.

mtcars %>%
  group_by(gear) %>%
  mutate(ave = mean(hp)) %>%
  ungroup() %>%
  group_by(cyl) %>%
  summarise(med = median(ave),
            ave = mean(ave), # should've named this variable something different
            sd = sd(ave)) # this is the sd of my newly created variable "ave", not the original one.

Try sd(data$var, na.rm=TRUE) and then any NAs in the column var will be ignored. Will also pay to check out your data to make sure the NA's should be NA's and there haven't been read in errors, commands like head(data), tail(data), and str(data) should help with that.