This is normally caused by editing the system crontab (/etc/crontab) and then using crontab(1) to install it:
# crontab /etc/crontab
This is not the correct way to do things. The system crontab has a different format to the per-user crontabs which crontab(1) updates (the crontab(5) manual page explains the differences in more detail).
If this is what you did, the extra crontab is simply a copy of /etc/crontab in the wrong format it. Delete it with the command:
# crontab -r
Next time, when you edit /etc/crontab, you should not do anything to inform cron(8) of the changes, since it will notice them automatically.
If you want something to be run once per day, week, or month, it is probably better to add shell scripts /usr/local/etc/periodic, and let the periodic(8) command run from the system cron schedule it with the other periodic system tasks.
The actual reason for the error is that the system crontab has an extra field, specifying which user to run the command as. In the default system crontab provided with FreeBSD, this is root for all entries. When this crontab is used as the root user's crontab (which is not the same as the system crontab), cron(8) assumes the string root is the first word of the command to execute, but no such command exists.