Warning message: In if (DataIfElse$Cost == "R") { : the condition has length > 1 and only the first element will be used

[Yarnabrina]

You are not using the same code as FJCC suggested. You have changed | to ||, and used Df$... instead of just column name inside case_when. You need not give dataframe name inside case_when.

Also, | and || are not same. The longer reports only one boolean based on 1st element, and the shorter returns a vector of booleans. I posted regarding this a while back, so I'm linking that below.

if_else not working as expected inside dplyr::mutate

Since OP asked why it happens, and not only how to rectify, let me add a small explanation to the answer of @valeri. @tlg265, if you see the documentation of logical operators (help("Logic", package = "base")), you'll see the following: & and && indicate logical AND and | and || indicate logical OR. The shorter form performs elementwise comparisons in much the same way as arithmetic operators. The longer form evaluates left to right examining only the first element of each vector. Evaluation proceeds only until the result is determined. The longer form is appropriate for programming control-flow and typically preferred in if clauses. In typical if situation, you'd want the cond…

Hope this helps.