After installing brew from brew.sh 

#install
brew tap mongodb/brew   
brew install mongodb-community

#setup database directory 
sudo mkdir -p /System/Volumes/Data/data/db
sudo chown -R `id -un` /System/Volumes/Data/data/db #if no errors good to go

#tell mongo new db path
sudo mongod --dbpath /System/Volumes/Data/data/db

mongo
mongod