Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers. This question, though, doesn't benefit from one.
RStudio Desktop works on my macOS, so the trouble probably is very likely that the underlying R program is missing. Think of RStudio as simply a graphical user interface to R. To install R, go to the CRAN download page for macOS.
If you are running Catalina, I strongly recommend downloading and installing from the notarized version. For earlier macOS, it may not be necessary.