ERROR: Could not find a version that satisfies the requirement torch<1.8.0,>=1.6.0 (from allennlp) (from versions: 0.1.2, 0.1.2.post1, 0.1.2.post2) code example

Example: ERROR: Could not find a version that satisfies the requirement torch===1.4.0

The most likely reason is a 32-bit installation of python, while 
the torch libraries rely on having a 64-bit version. 
Just start python from command line and observe

C:\Users\user>python
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 20:34:20) 
[MSC v.1916 64 bit (AMD64)] on win32

If it shows 32, then install 64-bit python