"open up a new command shell then type
set"
-- I did, and the environment variable I_MPI_ONEAPI_ROOT exists.
"cd into fds\Build\Scripts and type
setup_intel_compilers"
-- When I did that, I received the following error:
"*** Error: Intel compiler environment variable, IFORT_COMPILER, not defined."
" Intel compilers probably not installed."
Instead of double-clicking on the make_fds.bat file, I followed a workaround and was able to compile flawlessly.
I used the "Intel oneAPI command prompt for Intel 64 for Visual Studio 2019" and ran the make_fds batch file from there.
Seems like oneAPI needs to configure the environment for VS through its own command prompt each time, at least on my machine.