Cannot Install the Development version on MacOS 10.15

Summary:

I am trying to install the development version on MacOS 10.15 to make use of your survival analysis.

Description:

During the build process, if I skip the update I get the following error
In file included from /Library/Frameworks/R.framework/Versions/4.0/Resources/library/rstan/include/rstan/rstaninc.hpp:4:
/Library/Frameworks/R.framework/Versions/4.0/Resources/library/rstan/include/rstan/stan_fit.hpp:11:10: fatal error: 'stan/version.hpp' file not found
#include <stan/version.hpp>
^~~~~~~~~~~~~~~~~~

If I try to update rstan (2.21.1 -> 2.21.2) it errors out again:
module.cpp:2:10: error: 'stan/model/model_base.hpp' file not found with include; use "quotes" instead
#include <stan/model/model_base.hpp>
^~~~~~~~~~~~~~~~~~~~~~~~~~~
"stan/model/model_base.hpp"
In file included from Module.cpp:2:
./stan/model/model_base.hpp:4:10: fatal error: 'stan/io/var_context.hpp' file not found
#include <stan/io/var_context.hpp>
^~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

Reproducible Steps:

RStanARM Version:

2.21.1

R Version:

4.0.1

Operating System:

MacOS 10.15.6

Start by reproducing the development-version installation on macOS 10.15 with R 4.0.1 and RStan 2.21.1, then inspect the reported missing Stan headers during the build and RStan update. Done means the development version installs successfully without the listed header errors.