I am just guessing, but the Safari that iPad uses is not the full blown Safari that a Mac would use. I am guessing that there is a macro, or javascript, or something that is not going to run on the iPad version of Safari, and the error message is just giving you that one, as opposed to some other error message...
There are a lot of sites that don't display correctly, and many tasks that iPad Safari tries to interpret and basically does something funny (like asking for cookies to be enabled)..
If you mention the type of coupon software that you are using, others may have some ideas as to whether they have ever got it to work on their ipads..