In your code data_sel is just a character vector containing this values ("EE_2", "EE_4", "D1"), you can't apply the rename function over a character vector, I think this is what you actually want to do
library(dplyr)
ommerumfrage_Muenchen_2019_Haushalt <- read_csv("R/Quantitative Datenanalyse_SS2021/Sommerumfrage_Muenchen_2019_Haushalt.csv")
data_sel <- ommerumfrage_Muenchen_2019_Haushalt %>%
select(EE_2, EE_4, D1) %>%
rename(Nutzungsverständnis = EE_2, Bedienungsverständnis = EE4, Alter = D1)
data_sel
If this doesn't solve your problem, please provide a proper REPRoducible EXample (reprex) illustrating your issue.