if you read the instructions for running fairseq-py, they have a build step:

  
      
      GitHub
  
  
    

facebookresearch/fairseq-py 42

fairseq-py - Facebook AI Research Sequence-to-Sequence Toolkit written in Python.


  
  
    
    
  
  


See the section:
Install fairseq-py by cloning the GitHub repository and running: