I had the same issue at some point. I now have autocomplete working on my pass with .zshrc

From memory I think autocomplete worked for pass once I started using antigen.

I have the following plugins in my .zshrc plugins=( zsh-syntax-highlighting git iterm2 dotenv osx zsh-autosuggestions )

and also run antigen with user bundles for zsh-autosuggestions zsh-syntax-highlighting zsh-completions

Some magic somewhere in the above provided autocomplete for me with pass.

I hope it helps.