One thing that will block you from being able to add a .pkpass file to your Apple Wallet is the format of that file. I purchased a movie ticket that came with .pkpass file via email that included spaces in it so my phone was unable to recognize it (e.g., file name.pkpass).
To fix my issue I AirDrop’d it to my laptop, reformatted the file (e.g., file_name.pkpass), and sent it pack to my iPhone. It was instantly able to recognize it as valid format, and I successfully added it to my Apple Wallet.
Hopefully this will help future searches on this issue.