Brew cleanup Error: Permission denied @ unlink_internal

This worked for me:

brew unlink node && brew link node

First fix permissions causing the error:

sudo chown -R "$(whoami)":admin /usr/local

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On macOS Mojave, the chown command needs to be run on the specific directory, an example for the permissions issue in /usr/local/lib directory, this command would be

sudo chown -R "$(whoami)":admin /usr/local/lib

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Then relink node with the --force option as directed:

brew link --overwrite node