Thanks @michaelosthege for the suggestion that my Windows 10 machine may be having a clash of mingw compilers. As I mentioned earlier I use both R and Anaconda for my Statistics and Data Science work.
The Bayesian models I work with in R uses the Stan packages that need C and C++ compilers (via RTools that uses the mingw C and C++ compilers through Msys2). In the System environment variables, RTools is first on the PATH.
@michaelosthege had posted his USER Path variables and that showed that my Python compilation was not using m2w64 tool chain in Anaconda, but was using the compilers from RTools.
So I changed the name of the RTools folder and ensured that the System PATH variable would not work and the these mingw compilers could not be found. Then when I ran a simple command import theano as tp it gave me an error Error: g++ cannot be found...
Next I set up my User PATH variables and pointed them to the correct location of the mingw compilers in the Base Anaconda3 installation. All of my Pymc3 models are being compiled correctly.
Thanks everyone for your time and your help - Sree