First, test if you can actually run ANY crontabs. Create something that will do something that you can note - i. e. create a crontab like
echo test >text.txt
in BASH and see if at the time you set in the crontab, this file gets created.
If it does not work, find out if you have cron running at all.
If it does work, you might have a permissions problem - if I remember correctly you need to be root to add and remove crontabs. Also take a look at /var/log/cron (at least on RedHat 9) and see if the entries there indicate something is happening, or if there are error messages - I think you might find what you need there. Then, remedy whatever is posted as an error in the cron logfile (or post it here again) and see if it helps.