The provided information is not really enough to help you. I don't see
what is the relation between the "command not found" shell error and the
pip warning message below.
However, this seems a general problem of installing Python packages on
macOS, apparently with the stock Python provided by the OS. It is
unlikely to be an issue related to the architecture, more probably it is
related to the OS version. Asking for guidance on a Python users support
channel may be more appropriate.
I don't have any device running macOS 11, thus providing detailed
guidance is very difficult. However, if someone wants to send a M1
machine this direction, I would be more than happy to help sort out any
issue related to running Beancount on one such machine :)
Cheers,
Dan