I can't add categories to my shop

Hi,
I'm new to zencart, my problem is that when I add categories as shown in the tutorials, I get this error message;

Not Acceptable
An appropriate representation of the requested resource /zen/admin/categories.php could not be found on this serve

What am I doing wrong and how can I mend it, please :)

Thanks for your help.
Nic

Try reloading your /admin and /includes folders files using a "Good" FTP client like filezilla that reports errors

What words did you use when you tried adding your categories? Your server likely has a spam-protection rule in place (via mod_security) that's blocking the words you're entering.

Your server's errorlog should tell you the cause of the 406-not-acceptable error message.

(I'm inclined to think that an incomplete upload is not likely the problem, but it doesn't hurt to try if you think the upload might not have been "good")