I can't install Twitdroyd Pro for my Boost Samsung Galaxy Prevail, wtf?

Every time i download & try to install it always says "install was unsuccessful". Then i read you must remove the non pro version before you can use pro, one problem, my phone won't allow me to remove it. I'm using Boost's latest Android Phone, Samsung Galaxy Prevail, came out last month. Great phone. Anyone have anything on this? Thanks.