Mongo --nodb Command cannot find

$ echo $PATH

~/mongodb-osx-x86_64-enterprise-4.0.9/bin:~/mongodb-osx-x86_64-enterprise-4.0.9/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

$ mongo --nodb

-bash: mongo: command not found

May be your bash_profile needs to be updated

Or try to give full/absolute path of mongo/bin instead of relative path ~/mongodb

How did you install your MongoDB in OS X? Apart from that what was said in M001 course, take a look at general MongoDB documentation how to install MongoDB in OS X:

https://docs.mongodb.com/manual/tutorial/install-mongodb-on-os-x/