How create a crontab on mac?
This will indeed open a new crontab in vim (presuming you don't already have one)
crontab -e

If it creates a new crontab for you, it will start empty, hit the space bar to add something to the crontab before saving and exiting
So TL;DR

crontab -e creates a new crontab if you don't already have one
If it's a new (empty) crontab, press i to 'insert' using vim, then press spacebar a few times so it's not empty
Save and exit vim by pressing these keys (in order): esc, :, w, then q