@alfroc you might want to check the KNIME preferences …
And maybe also the idea with the Conda environment propagation (Data separate and melting - #9 by mlauber71) if that does not work. It will not be enough to have R running on the system. You will have to tell KNIME (and maybe the R node) where to find it.
Also there are two R integrations. The community one would need R(serve) to run outside before you use it. But I assume you have not used that.
Also the guide to KNIME and R is highly recommended if you have problems:
https://docs.knime.com/latest/r_installation_guide/index.html