This issue appeared after upgrading macOS to Mojave 10.14.X onwards.
Therefore, you need to reset the permissions in /usr/local:
sudo chown -R $(whoami):admin /usr/local/* \
&& sudo chmod -R g+rwx /usr/local/*
Source: https://github.com/Homebrew/homebrew-core/issues/45009#issuecomment-543795948