Zsh: illegal hardware instruction error when running mix local.hex

I am using asdf to install elixir, erlang , nodejs and rust.
Using this link as a guide.
https://hexdocs.pm/phoenix/installation.html
https://astonj.com/how-to-set-up-a-ruby-and-elixir-dev-environment-on-macos-2021/
(excluding the ruby instructions)

When I run mix local.hex I get

zsh: illegal hardware instruction mix local.hex

on macbook pro 13" 2009 upgraded to Catalina with dosdude1 Catalina patcher.
http://dosdude1.com/catalina/

echo $0 outputs zsh
googled zsh: illegal hardware instruction mix local.hex and found a few links…

The problem might just be Catalina is not going to be reliable on an old 2009 MBP. But just in case someone might be able to send me in the right direction - anybody seen this problem before?