No I'm still getting the same error, but I think I found why it is giving me the error.
I have purchased all of cherrykissgames' visual novels, and still have them installed so I deleted all of them. I tried to install Stealing a Monster Girl Harem again, and it finally let me install it. I started re-downloading all of the other visual novels, and reinstalling them. When I got to Monster Girl Breeder it was giving me the same error. I than went to the downloaded apks in my download folder I noticed that the names for Monster Girl Breeder (com.cherrykissgames.monsterharem-1.2.5.apk), and Stealing a Monster Girl Harem (com.cherrykissgames.monsterharem-1.1.7.apk) were exactly the same except for the version number (1.1.7 & 1.2.5). I tried renaming one to see if that did the trick, but I still got the same error. So I believe that both apps are somehow being recognized as the same, which is why I'm getting the error of it being an update.
I have sent them an email, and hope I get a response soon. I know I could just delete one of them when I'm done playing it, but I kind of like leaving it installed if I ever want to go back to it.