I see rstan downloaded and installed inside rstudio. Is that what you’re asking?
I reinstalled R, Rstudio, and rstan today as well just in case something was corrupted or installed weird. But I had the same error after the re-installation of all three.
No, in RStudio you have an “environment” window in the top right of your screen (under default settings). It contains all the variables that you have already declared. Before running the line that gives you trouble, you should check that there is a variable called “stan_program” there.
Right now your code calls the function “stan” using the argument “model_code = stan_program”. So the object “stan_program” must already exist (and be of the type that this function expects).