I faced the same issue and tried all the solutions that folks suggested here and other links (like https://askubuntu.com/questions/695981/platform-not-supported-for-tensorflow-on-ubuntu-14-04-2).
It was so frustrating because using
print(wheel.pep425tags.get_supported())
I could see that my ubuntu supported ('cp37', 'cp37m', 'linux_x86_64') and that was exactly what I was trying to install (from https://storage.googleapis.com/tensorflow/linux/cpu/tensorflow-1.14.0-cp37-cp37m-linux_x86_64.whl).
What at the end fixed it was to simply download the package first and then
pip install tensorflow-1.14.0-cp37-cp37m-linux_x86_64.whl