Visual Studio 2015 doesn't have cl.exe

Visual Studio 2015 doesn't install C++ by default. You have to rerun the setup, select Modify and then check Programming Language -> C++

For me that have Visual Studio 2015 this works:
Search this in the start menu: Developer Command Prompt for VS2015 and run the program in the search result.
You can now execute your command in it, for example: cl /?

In Visual Studio 2019 you can find cl.exe inside

32-BIT : C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.20.27508\bin\Hostx86\x86
64-BIT : C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.20.27508\bin\Hostx64\x64

Before trying to compile either run vcvars32 for 32-Bit compilation or vcvars64 for 64-Bit.

32-BIT : "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars32.bat"
64-BIT : "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvars64.bat"

If you can't find the file or the directory, try going to C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC and see if you can find a folder with a version number. If you can't, then you probably haven't installed C++ through the Visual Studio Installation yet.