You may have to put it off a few days. Depending on the prep, you may be able to mix it with an allowable liquid (Lucazade in certain colors is an option) that might make it more tolerable. Having it at a really cold temperature might help as well. If all still goes poorly with the prep, the doc may prescribe a pill-based regime or an alternative liquid prep and try again.
Most importantly, be honest with your doctor about the prep. It can be at best a waste of time to do the scope if you aren't really prepped, and at worst it can be dangerous. Best of luck with it - if you haven't had one before the prep is BY FAR the worst part of it.