These things come to my mind:
- do you have the package dplyr installed? Maybe you check in your R version (with RStudio it is easy to do so)
- Make sure you have the latest version von Rserve installed (cf. R Problems on macOS (high sierra) since R version 3.5) and also preferably R 3.5.1
- does your KNIME installation point to the correct R version. You can install R ‘within’ KNIME from the extensions and you can have a separate R version on your system and just ‘point’ KNIME to that - it is easy to confuse the two. I like just to have my own R version separate from KNIME.