Error: Cannot install in Homebrew on ARM processor in Intel default prefix (/usr/local)

For what it's worth, before installing Homebrew you will need to install Rosetta2 emulator for the new ARM silicon (M1 chip). I just installed Rosetta2 via terminal using:

/usr/sbin/softwareupdate --install-rosetta --agree-to-license

This will install rosetta2 with no extra button clicks.

After installing Rosetta2 above you can then use the Homebrew cmd and install Homebrew for ARM M1 chip: arch -x86_64 /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

Once Homebrew for M1 ARM is installed use this Homebrew command to install packages: arch -x86_64 brew install <package>

Homebrew needs to be installed in two places on Apple silicon: in /usr/local for rosetta-emulated (Intel) code, and /opt/homebrew for ARM64. These are somewhat hard-coded and the /opt/homebrew one MUST be used for ARM code, as it stands today, and is non-negotiable. However, it's easy enough to install and you can follow the general instructions on the official docs. You open a Rosetta shell first.

% arch -x86_64 zsh
% cd /usr/local && mkdir homebrew
% curl -L https://github.com/Homebrew/brew/tarball/master | tar xz --strip 1 -C homebrew

Later, to install tools under intel you can use the arch command per-command or open a zsh terminal as above.

arch -x86_64 /usr/local/homebrew/bin/brew install sometool

For the ARM architecture, you don't need to use arch but you do need to use the full path of the homebrew command.

/opt/homebrew/bin/brew install sometool

You could rename the brew commands to something like brew86 and brewARM to help keep them separate.