The package cron_3.0pl1-84 which enter Sid on 29th July is broken. The file /usr/bin/crontab is missing. The bug was reported as #261897 and fixed.
Sid player should upgrade to cron_3.0pl1-86 or above.

If you had already upgrade to this version, you can add the following line in your /etc/apt/sources.lists:

  deb http://snapshot.debian.net/archive pool cron

To rollback to cron_3.0pl1-83 (i.e. apt-get update and apt-get install cron=3.0pl1-83)