I was setting up a cron job running under a newly created account, and when I edited the crontab with

# crontab -e pch

I was told

Warning - Invalid account: 'pch' not allowed to execute cronjobs

The first thing I did was create /etc/cron.d/cron.allow and put the username in there. Didn’t work. (I should have known that cron.allow only applies if it exists - that is to say that if it doesn’t exist, any user not in /etc/cron.d/cron.deny can run jobs.)

It took me a while to work out what was wrong, which is why I’ve written it down here. The user’s password entry in /etc/shadow was *LK*, which means the account is locked. Locked accounts can’t run cron jobs. So I changed that to NP, which means "no valid password" and all was well.