Hi!
To help us help you, could you please prepare a reproducible example (reprex) illustrating your issue? Please have a look at this guide, to see how to create one:
You need to create column "c1" before you can replace it. For example
data$c1 <- NA
or you create it and fill it at once, like this
data$c1 <- ifelse(data$x %in% c("anxifree", "placebo"), -0.5, NA_real_)
Also you have spelled placebo wrongly.
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